We had some problems lately with the TB16 dock, unfortunately we are now running our XPS 9560 without the dock.
This means our display port that supported 3440 x 1440 60Hz is also gone.
Our LG 34UC99 is now running over the HDMI port in the laptop itself.
The maximum output resolution is 3440 x 1440 50hz.
It feels like there is some lag or 'judder' when the mouse moves.
It's like the mouse is not exactly doing the thing you want it to do.
It is very hard to describe, but the difference is very strong with the 3440 x 1440 60Hz displayport.
The USB-C port (Thunderbolt 3) is connected with our QNAP NAS so we can not connect our screen that way.
Is there any solution to get the 60hz smoothness over HDMI?
I think the refresh rate change is the culprit. 10 Hz is more than a 15% drop, and I can see that being noticeable. If you want to confirm, temporarily connect the display back the way it was and force it to run at 50 Hz while connected that way to see if you observe the same behavior.