I'm not really sure how to best phrase the subject, but:
The situation is I have a laptop connected to an external monitor. When I use the laptop's own display, everything is fine. But today, I've found that when I use the monitor, the screen just... slows down. That is, scrolling in a window is laggy, the desktop animations are jerky (eg when minimising and maximising a window), the responsiveness when typing this post is laggy, etc. This is all in the Windows desktop, not games or whatever.
The laptop is a Lenovo X1 Extreme running Win 10 Pro 1903, and the monitor is a 7-year-old Dell P2715Q 4K unit, connected via a DisplayPort-to-USB3 adapter. GPU-Z doesn't report any issues with the card, as far as I can tell, and Windows also seems to be working properly. The monitor has been having issues for some time; often it refuses to wake from sleep unless I physically unplug the power cord and plug it back in again.
Any ideas what could be causing this, and what I can do to fix it? Is it time to just bite the bullet, and get a new monitor?
ETA: Windows still shows the monitor running at 4K/60Hz, so the USB port seems to be supplying enough bandwidth.
ETA2: the connection is actually Thunderbolt, behind a USB-C port. I've also found that everything works correctly if I duplicate the display on both the laptop and the monitor. If I then switch back to using only the monitor, it continues working for a few seconds, and then slows down again.
Device Manager also doesn't show any problems, and all my drivers are up to date as far as I know.