Took me a while to find out that just because my Thinkpad Notebook supports DisplayPort 1.2 and has Thunderbolt, it does not mean I can actually use the specified resolutions with the Thinkpad USB-C Dock Gen 2.
I don't know if you have every used 30Hz with a mouse, but it sucks. Especially when you are used to 120Hz... Now I am looking for a Dock that actually supports 4k60Hz with DisplayPort 1.2.
I've tried several docks and cables with no success until I've found this article: https://www.bigmessowires.com/2019/05/19/explaining-4k-60hz-... The recommended CableMatters dock solved the 4k60Hz issue with USB2.0 pheripherals connected (mouse, kb).
Generally usb-c docks can do 4k@30 but to get 60 you need a (any) thunderbolt dock. The TB port on the laptop can do 60 but you need a TB dock and not a USBC dock otherwise that would be the bottleneck. However if you just need video and not extra USB ports / Ethernet / etc., then a USB-C to DP adapter can do 60fps since it doesn't need to reserve bandwidth for the other stuff.
For reference, DisplayPort 1.2 supports 4k60Hz according to Wikipedia: https://en.wikipedia.org/wiki/DisplayPort#Refresh_frequency_...
But when I look into the User Guide of the ThinkPad USB-C Dock Gen 2, the max resolution for DisplayPort 1.2 is 4k30Hz (Page 7): https://download.lenovo.com/consumer/options/thinkpad_usb-c_...
I don't know if you have every used 30Hz with a mouse, but it sucks. Especially when you are used to 120Hz... Now I am looking for a Dock that actually supports 4k60Hz with DisplayPort 1.2.