Home setup was a one point a MacBook Pro 15" and has since been upgraded to a MacBook Pro 16", with the laptop running in clamshell mode when connected to a Dell monitor; USB-C to display port cable. Worked well - scrolling was smooth on the Dell monitor.
Added a laptop stand and then a Twelve South StayGo to the mix so I can utilize both the Dell monitor and the MacBook Pro display at the same time. The connecting cable is now HDMI to HDMI from the Stay Go to the Dell Monitor. Scrolling on the MacBook Pro is fine, but the Dell is a little choppy.
Is it possible this a systems preferences setting, a setting on the Dell, or is this a result of using a different cable? Is the lag possibly from going through the Stay Go?
Check to find out the refresh rate of the monitor. I notice my display behaving as you describe when it’s running at 30Hz instead of 60Hz. Most (all?) USB-C (not Thunderbolt 3) HDMI adaptors can drive a 4K monitor at a maximum refresh rate of 30Hz and that seems to be the case for the one you bought.
Thanks for the note - and I see your cool flag with your username … I am a Canadian citizen but a US resident these days 
Just checked the monitor: P2715Q 27" Ultra HD 4K Monitor, and it has a refresh rate 60Hz. I went HDMI to HDMI because the Stay Go doesn’t have a USB-C port I can use. Let me see what happens if I direct connect the monitor to the laptop again.
I was trying to have one cable connection only but maybe that’s not going to be possible.
1 Like
Just realized something else - it is possible the HDMI cable is ‘older’ and not able to handle the throughput appropriately?
The monitor can be capable of 4K@60Hz, but I’m pretty sure that adaptor isn’t (according to the Twelve South website). You can check to see what it’s actually running at in the Graphics/Displays section of System Report.
Also, hi from up north (and west) 
I had the same problem earlier after changing a cable. For some reason it went to 30hz. Hold down option and click on scaled in the display preferences and it will give you all the refresh options. In my case I was able to change to 60hz from there and all good which was ideal as couldn’t wait another week for a new cable!
Got a reply back from Twelve South who stated:
The frame rates are as follows:
1080p @ 60Hz
4k / 2k @ 30Hz
Going to try the solution @Ethan9482 suggested.
That’s a shame, I didn’t look at the spec of the cable but if thatIs saying 30hz at 4K you might be out of luck. However if the monitor is capable at least it’s a relatively cheap fix.
It’s been surprisingly difficult to find USB-C 4K 60Hz capable adaptors, without going to Thunderbolt 3 docks. It looks as if there are a few available now.