iMacs and internet connection bandwidth

[PBMB]

I have the following problem with two iMacs, a 21.5-inch model under Big Sur and a 27-inch model under Monterey. Both are reaching only 50-70% of the available wireless internet connection bandwidth. The proof that there is no loss due to the Wi-Fi is that at the same spots where the iMacs are installed, a Windows laptop and an iPad mini 6 reach the full available bandwidth with the same wireless connection and the same bandwidth measuring utility (Speedtest application by Ookla).

So the problem is with the iMacs. I tried disabling Little Snitch and Firewall but to no avail. Nothing changed. At this point I don't see what else I could do.

Any ideas? Thanks!

 

[Slartibart]

Compare the MTU size set on your Windows laptop to the one used on your Macs. Make sure the Macs use the same Ghz-band on your router as the other devices.

 

[PBMB]

Compare the MTU size set on your Windows laptop to the one used on your Macs. Make sure the Macs use the same Ghz-band on your router as the other devices.

All the computers tested, including the iPad, use the same GHz-band. But I don't know what the MTU size is and how to check and change it, if needed.

 

[Slartibart]

Maximum Transmission Unit - you find it in the network hardware settings on MacOS.

The MTU size impacts on the network performance - a simple example is giving here.

In theory large MTU sizes allows the operating system to send fewer packets of a larger size to reach maximal network throughput - and larger packets greatly reduce the processing required in the OS.

On asynchronous transfer mode adapters, the default MTU size of 9180 is much more efficient than using a MTU size of 1500 bytes.
With only e.g. Gigabit and 10 Gigabit Ethernet it would be best to use jumbo frame mode.

Smaller MTU sizes result in lower latencies while increasing processing in the OS.

But all this depends on all network hardware - e.g. your router firmware might not support certain parameters and/or the WLAN/LAN connections become unstable.




TL;DR: try to optimize the throughput of both affected Macs by increasing/decreasing the MTU size. Take a look on your router settings.



EDIT:corrected semantic error by typo 😇

 

[PBMB]

Very informative, thank you! I tried to change the MTU value in System Preferences but in manual mode it does not allow more than the automatic size which is 1500. More precisely, I can set the MTU only between 1280 and 1500. Playing a little bit in this range does not seem to make any difference. This is the same for both iMacs.

I also tried to delete the Wi-Fi connection from the Network preferences and to forget it from the list of preferred networks in the "Advanced ..." section. No change in the Speedtest results. The iPad mini still goes strong at full bandwidth.

 

[PBMB]

Here are more tests with older Macbooks Pro. A 15-inch model from 2014 running Mojave achieves full bandwidth in one go, like the iPad mini. Another 15-inch model from 2009 running El Capitan reaches 80% of the available bandwidth but I think in this case there are several limitations anyway (Core 2 Duo and slow spinning hard drive being the most notable). But even this ancient machine performs better than the iMacs. In all cases the MTU size is set automatically at 1500 (which is the maximum value), the computers are on the same Wi-Fi band (5 GHz), the portable computers are placed close to the iMacs during testing and the measurements are carried out using the Speedtest application by Ookla.

However, seeing discussions like this one, makes me wonder if there is indeed a deeper problem with the OS itself. These iMacs were performing correctly in the past but I did not check their network performance for a long time. So I cannot exclude that the problem is due to a system update. I applied the last macOS updates but this made no difference. It is really puzzling.

 

[PBMB]

After extensive testing here are my findings (and the solution).

I tried to reset the PRAM and the SMC. No change after reboot. I tried then to start up in safe mode. Here is where it gets interesting.

On the 21.5-inch iMac: while in safe mode, I logged in to the most frequently used account and I ran Speedtest. Result: nearly 100% of the available bandwidth. The results were consistent across the other accounts as well after rebooting normally.

On the 27-inch iMac: I applied the safe mode trick but I did not log in to any account, just restarted again. Then I logged in to a test account and ran Speedtest. The result was as in the 21.5-inch iMac, around 100% of the available bandwidth. I rebooted in normal mode, I tried Speedtest on the most frequently used account and then ... again drop to 50-70%. Then I started again up the iMac in safe mode and I logged in to the same "problematic" account. I ran Speedtest and everything went smoothly (100% speed). I rebooted again in normal mode and ran again Speedtest for the other users too. Solid 100% everywhere this time.

I don't know what precisely safe mode does but this shows that there is some kind of system cache cleaning. What I don't understand is how one user's settings or whatever can affect internet performance for other users. My guess is that there may be some setting/cache corruption at system level that can be reflected to other users too; and to get rid of it, you have to not only boot in safe mode, but to log in to the culprit account while in safe mode in order to purge.

Anyway, problem solved .

 

[Fishrrman]

OP:

When you start in safe mode, you are preventing all 3rd-party extensions from loading at boot.

Suggestion:
Go to the users & groups preference pane, and create a NEW account (which will be "for test purposes only").
Give it administrative privileges.
Don't set up anything except a username and password. You want this to be "bare bones".
Then, log out of your "regular" account and log into the test account.

Now try speedtest again.
DO NOT "safe boot" for this.
What kind of results do you get?

If you get 100% or so of the bandwidth, then it could be something "localized" in your "regular" account(s) that is slowing you down.

 

[PBMB]

OP:

When you start in safe mode, you are preventing all 3rd-party extensions from loading at boot.

Suggestion:
Go to the users & groups preference pane, and create a NEW account (which will be "for test purposes only").
Give it administrative privileges.
Don't set up anything except a username and password. You want this to be "bare bones".
Then, log out of your "regular" account and log into the test account.

Now try speedtest again.
DO NOT "safe boot" for this.
What kind of results do you get?

If you get 100% or so of the bandwidth, then it could be something "localized" in your "regular" account(s) that is slowing you down.

Well, I tried it and I obtained about 100% of the bandwidth. But this is not surprising because as I said previously, logging in to each account in safe mode, purges something, or so it seems, and the problem disappeared. So, with this problem already solved, testing a new, bare-bones admin account under normal boot conditions should give the normal behaviour. And so it did, as expected.

It would be interesting to see what this experiment would give if the problem still persisted. I will definitely try it if this problem resurfaces again.

 

[PBMB]

It looks like I spoke too soon. The problem reappeared on the 27-inch iMac and I already tried the experiment suggested above, that is to create a bare-bones admin account and to test the internet connection from there. The result may look surprising but even from the new account the internet speed runs at 50-70% of its potential.

Then I made an accidental discovery. I ran Speedtest from a test account, not a fresh one, while I was logged out from the main account (non-admin) that I have for everyday's work, let's say "account W". And ... surprise, Speedtest showed the full available bandwidth.

More testing: I ran again Speedtest from all accounts while I was logged in to the "account W". In all cases I had this large drop in speed. Then I logged out from "account W" and repeated the test. Result: full bandwidth in all cases.

Therefore there was something in "account W" that slowed down the internet speed for all other accounts, which sounds a bit incredible. I logged in to the "account W" but I closed all applications before running Speedtest. Result (with all applications closed): full bandwidth.

OK, I was near the solution. By trying one by one all application that were running before in "account W", I found that the culprit is Desktop Clock Live by Voros Innovation & Business Services (there is no direct link, I downloaded the free version from the App Store). I could now reproduce the bug: if that application is open and running, then the internet speed drops as described for all user accounts. If it is closed, then the internet speed is fluctuating around 100% (98-101%).

I still cannot understand how such a simple clock application can cripple the internet speed for all users in the computer it runs. Unbelievable.

 

[Fishrrman]

Good problem solving by the OP...

 

[Jumpthesnark]

Great work, OP. And thanks for the warning about the spammy/scammy app that is somehow degrading bandwidth speeds. There are several App Store reviews of the app that indicate it is buggy in many ways, including finder freezes and more. Clearly there's more going on with the app than running a simple clock.