Monitors usb-c DP alt mode 30 Hz only when USB 3 speeds

[Lee Cooper]

Seems to be an issue with newer monitors advertising usb-c DP alt mode:
- if run at 60Hz, the usb speed drop to usb 2, around 30Mbps for a SSD connected to one of the monitor ports.
- if run at 30Hz the usb speed is "high speed" around 500Mbps.

This behavior makes the usb hub on the monitor practically useless when the monitor is used at 60Hz, maybe good for a mouse or keyboard.

I found postings on slow usb speeds on the new DELL monitors U2723QE but also on more high end such as EIZO CS2740.

This problem is not reported on monitors with Thunderbolt 3 ports.
What could be the issue?
Eizo actually stated this in the user manual:

 

[Amethyst1]

What could be the issue?

The issue is present if the monitor, and/or the Mac it's connected to, only supports DisplayPort 1.2.
A four-lane DisplayPort 1.2 HBR2 connection is necessary for "4K" at 60 Hz, so all four of USB-C's high-speed lanes need to be dedicated to DisplayPort Alternate Mode, leaving no high-speed lanes for USB 3.x, so USB speeds are limited to 2.0.

If the monitor and the Mac it's connected to support DisplayPort 1.4, things are different.
A two-lane DisplayPort 1.4 HBR3 connection is sufficient for "4K" at 60 Hz, leaving two of USB-C's high-speed lanes available for USB 3.x speeds.

This problem is not reported on monitors with Thunderbolt 3 ports.

Thunderbolt 3 has enough bandwidth for four lanes of DisplayPort plus USB 3.x speeds, so you can get "4K" at 60 Hz even if you're limited to DisplayPort 1.2.