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.

  • My bet is the USB adapter can't keep up with the 4K resolution – Twisty Impersonator Jun 22 at 1:02
  • @TwistyImpersonator I've actually been using this setup for a few months; it only went bad today. – Hong Ooi Jun 22 at 1:09
  • I see. Worth putting into the question. – Twisty Impersonator Jun 22 at 1:38

Technically, there is no "problem." The issue the USB speed.

Are you using you laptops USBv3 port and is your USB to DP adapter USBv3? There are USBv3 versions, which are 10x faster than USBv2. However, I have not used a USBv3 adapter and cannot say how much better they are. What I do know, is the USBv2 versions simply arent even close to fast enough for anything other than writing emails, documents, spreadsheets, etc. I would strongly feel USBv3, even though they are 10x faster, wouldnt be fast enough. Running at 4K resolution isnt helping either.

  • Yes, it's all USB3. I was using this setup for a few months without any problems. It only went bad today. – Hong Ooi Jun 22 at 1:48
  • @HongOoi You didnt make that clear in your question. Obviously, double check that the USB port is running at v3. I havent used these adapters in a very long time, but in the past, the were known for going bad. Perhaps this adapter has gone bad as well. – Keltari Jun 22 at 1:50
  • Per the edit, Windows still reports that the monitor is running at 60Hz, which would imply the port is supplying enough bandwidth – Hong Ooi Jun 22 at 1:56
  • @HongOoi That is incorrect. 60Hz the refresh rate, which has has NOTHING to do with the bandwidth. – Keltari Jun 22 at 2:22

A BIOS update showed up in Lenovo Vantage a couple of days after I posted this question. Installing that seems to have fixed the problem.

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