Scaled external 4k monitor randomly slow

[dandymac]

Hi
I just bought a DELL U2720Q 4K. Tried at 3840x2160 but it's too small so I scaled it down to 2560x1440 and it goes well, no slow.

After I shutdown and turn on the monitor, it was back to 1920x1080, strange. so I'm going to rescaled but now I noticed that hovering the first option now it doesn't show dock and window in the preview on the left, neither the resolution 1920x1080 as it shown before.

Here, hovering first option:

Hovering second option now it show the 1920x1080, and third one the 2560x1440. So it seems all the options are translated.

Then, I rescaled to 2560x1440 (third option, while before was the second one) and now it's slower than before. And I can't get it back to works as before without lags. I especially notice the slowness when scrolling webpages, moving cursor, dragging windows, ...

You know what's happened?

 

[robbieduncan]

Please provide details of your hardware. Some Macs will struggle with non-integer scaling. So 2x will be fine but 1.5x will be slow/juddery

 

[dandymac]

Hi @robbieduncan
Here my hardware:

MacBook Pro (15-inch, 2018) with macOS Catalina 10.15.4
CPU 2,6 GHz Intel Core i7 6 core
RAM 16 GB 2400 MHz DDR4
GPU:
Radeon Pro 560X 4 GB
Intel UHD Graphics 630 1536 MB

Remembering that the first time I plugged the monitor it works very well and fast. So it seems that the problem is not related to Mac hardware...?

 

[robbieduncan]

I would expect the Radeon Pro to handle this very well! I was expecting a Mac Mini with Intel graphics only. Do you have the Radeon turned off via performance settings perhaps? Does this behaviour persist after a reboot?

 

[dandymac]

Does this behaviour persist after a reboot?

Yes 🙁

Do you have the Radeon turned off via performance settings perhaps?

@robbieduncan Where to check? This?

 

[robbieduncan]

The very first checkbox on the page can stop the switching on battery. I'd try turning that off. Otherwise has anything changed since it worked well? Software updates? Any software installs?

 

[dandymac]

@robbieduncan tried to turn off but frankly it's the same.
Nothing changed! Today I just turn off and turned on the monitor and yesterday worked well!

 

[robbieduncan]

There is some strange things in there. Why is it only connecting at 29Hz? Do you have a second cable you can try? I have 2x4k displays connected and I see both connected at 60Hz (as expected). Also why is your GPU only connecting on a x8 bus? I would expect x16 although I do not expect this makes any real difference. What OS version are you running?

 

[dandymac]

Well @robbieduncan,

Why 29Hz? I really don't know!

Also why is your GPU only connecting on a x8 bus? => Really don't know what it means

OS version: macOS Catalina 10.15.4

I'm using the USB-C-->USB-C cable included with the monitor. I can try another cable DP-->USB-C. Anyway the monitor at some point worked well and I was using the USB-C-->USB-C cable.

Note: USB-C-->USB-C cable works also for recharging the Mac, so I don't use the Mac adapter but just that cable

 

[dandymac]

@robbieduncan I tried with another cable, it's a DP-->USB-C cable and now it' a lot more reactive and fluid.
Here the screen:

Is it all okay at your eyes now?

To me there's something strange yet. Check how changed these details:


With USB-C-->USB-C cable
Risoluzione: 5120 x 2880 (5K UHD+ - Ultra High Definition Plus)
Aspetto interfaccia utente: 2560 x 1440 @ 29 Hz


With DP-->USB-C cable
Risoluzione: 3840 x 2160 (2160p/4K UHD 1 - Ultra High Definition)
Aspetto interfaccia utente: 1920 x 1080 @ 60 Hz


With DP-->USB-C cable the monitor is a lot faster but it seems a less defined (or maybe a little little bit blurry).

Possible?

 

[robbieduncan]

60Hz is better. Running soother is better. But the virtual/apparent resolution has dropped (was 2560x1440, not 1920x1080). Perhaps the USB-C->DP cable is limited to 1080p? This is also now an integer scaling factor which would be faster Too. Is this the highest resolution you can choose with that cable?

 

[dandymac]

The cable is this one and it seems for 4K...

With this cable I can choose the same resolutions as before, indeed now I'm using 2560x1440, as before, but now it's more faster.

So strange... what's going on!?

 

[robbieduncan]

I have to admit I am not 100% sure! I am just following general trouble shooting. It might suggest the USB-C cable is not 100% and perhaps not sustaining full data transfer rates. I am not sure how to test that though

 

[dandymac]

Take a look at these:

Case 1: With USB-C-->USB-C cable
Risoluzione: 5120 x 2880 (5K UHD+ - Ultra High Definition Plus) ==> why 5k? the monitor is 4k
Aspetto interfaccia utente: 2560 x 1440 @ 29 Hz ==> correct choosen resolution but strange Hz... why?


Case 2 (actual): With DP-->USB-C cable
Risoluzione: 3840 x 2160 (2160p/4K UHD 1 - Ultra High Definition) ==> these seems correct, right? it's a 4k monitor
Aspetto interfaccia utente: 1920 x 1080 @ 60 Hz ==> this is strange because I choose 2560x1440, correct? And yet the resolution is very similar (if not equal) to before...


What do you think? Could you check your data? How they are compared to the above ones?

 

[dandymac]

Just now, with the same DP cable, without changing nothing:

SO randomly.

I this supposed to be the correct data I should have?

 

[dandymac]

I get this answer by Dell:

The aspect ratio or the resolution will depend on the window or application that you are currently opening. As some applications may not run in 4K as they were no developed to run in those resolution. Depending on the system that you have connected to the monitor, the resolution will get automatically switch to one that will allow to run the application with no problems.

As for why it only correctly works with the USB C to Display port, that's because this monitor works with dongles or adapters while using that port and is not going to perform correctly when using a USB C to USB C cable.

You can check this page under the tech specs for this monitor.

The part where it says: 1 x USB Type-C (Alternate Mode with Display Port 1.4, USB 3.1 upstream port, Power Delivery upto 90W*).

When we say that USB C to USB C is not recommended, that's because this type of port will not give you as much resolution as the Display port connectivity.

There are some situations that can be related to the system being an Apple unit, these monitors have not been tested on those units.

The USB C is usually meant for docking stations or other USB devices, that's what it meant for Power delivery.

As we explained you previously, we are not saying that USB C to USB C does not work, it just means that to avoid having the slowness and dragging that you are reporting, it's recommended to use the USB C to DP.

Are they kidding me? So connection with USB C to USB C cable doesn't work by default?

 

[steffi]

I have the same monitor with Vega Pro 20 over USB Type C and System Report shows me

Displays:


DELL U2720Q:


Resolution: 5120 x 2880 (5K/UHD+ - Ultra High Definition Plus)
UI Looks like: 2560 x 1440 @ 60 Hz
Framebuffer Depth: 30-Bit Color (ARGB2101010)

Main Display: Yes
Mirror: Off
Online: Yes
Rotation: Supported


Automatically Adjust Brightness: No


Connection Type: Thunderbolt/DisplayPort

 

[Erehy Dobon]

According to Dell's webpage for the monitor in question:

Computer Accessories and Peripherals

Shop Dell’s high-quality computer accessories like keyboards, mice, webcams, batteries and more. Enjoy great deals and build your ideal PC setup for home or business.

www.dell.com

it is a 4K monitor (3840 x 2160), not a 5K monitor.

Thus, connection via USB-C is A.) not connecting at the right frequency and resolution by default, and/or B.) not reporting correctly what the connection is.

I have seen a similar problem in the past using an HDMI cable connecting my Mac mini 2018 to a 27" LG HDR UHD (4K) monitor and the cable did not report the correct connection. I am currently connected via a USB-C cable between the two.

I suggest you try explicitly setting it to 3840 x 2160 resolution. If you press the Option key when you click on "Ridimensionata" you might get frequency options; try 60 Hz. It may be better to select "Di default per monitor" (native resolution).

I have a Mac mini 2018 with integrated graphics driving the LG monitor at 4K @ 60 Hz and your MacBook Pro's discrete graphics are better than mine.

 

[dandymac]

I suggest you try explicitly setting it to 3840 x 2160 resolution. If you press the Option key when you click on "Ridimensionata" you might get frequency options; try 60 Hz. It may be better to select "Di default per monitor" (native resolution).

That! 🙌

And this post could help a lot of people with this monitor:

U2720Q, MacBook Pro, 29Hz? | DELL Technologies

Hi I've this Macbook Pro: MacBook Pro (15-inch, 2018) with macOS Catalina 10.15.4CPU 2,6 GHz Intel Core i7 6 coreRAM 16 GB 2400 MHz DDR4GPU:Radeon Pro 560X 4 GBIntel UHD Graphics 630 1536 MB tha...

www.dell.com

 

[eyalben]

WOW!
I was about to complete my purchase of this model because it sounded so perfect (USB-C Hub + Display + PD + 4K)
Good thing I decided to search the model on this forum right before completing the purchase.

If the USB-C connector is useless, I can go for the cheaper models.

 

[macsnack]

That! 🙌

And this post could help a lot of people with this monitor:

U2720Q, MacBook Pro, 29Hz? | DELL Technologies

Hi I've this Macbook Pro: MacBook Pro (15-inch, 2018) with macOS Catalina 10.15.4CPU 2,6 GHz Intel Core i7 6 coreRAM 16 GB 2400 MHz DDR4GPU:Radeon Pro 560X 4 GBIntel UHD Graphics 630 1536 MB tha...

www.dell.com

Hey @dandymac! Just to confirm that all issues are resolved now? I am thinking of this over the LG 4K as well, though my use case is with my iPad Pro and sometimes with my 13 MacBook Pro.

 

[eyalben]

Hey @dandymac! Just to confirm that all issues are resolved now? I am thinking of this over the LG 4K as well, though my use case is with my iPad Pro and sometimes with my 13 MacBook Pro.

Just got the Dell U2720Q for testing before purchasing.

I can confirm it works great with the MacBook Pro 16" via USB-C:

1. USB-C is charging the MacBook Pro (90W)
2. It is using 60Hz but requires scaling down (default is HUGE)
3. Works as USB hub (I have both speakers and USB mouse connected to the monitor)

I have enabled the following settings:
- USB-C charging is always on
- Display -> USB-C Prioritization: High Data Speed

It looks like there is a bug in System Information: when changing the 'Scaling' under the display preferences,
it reports weird 'native resolution' values, sometime it shows the display is detected as 5K, sometimes even more.

The monitor always reports that the input resolution is 4K so the mac output is OK scaling only changes the DPI.

 

[macsnack]

It is using 60Hz but requires scaling down (default is HUGE)

Thanks for answering this, @eyalben!What option are you using in Display Settings?

 

[eyalben]

Thanks for answering this, @eyalben!What option are you using in Display Settings?

By default, mac uses half the vertical/horizontal resolution - so 4K display will become 2K (1920x1080) with high-DPI.

I am using scaling of 2560x1440 which might cause some performance issue (though I had no issues at all!)
(It is actually renders a 5K image and scales it down to the 4K display).