I can confirm I have the same issue with my i5 version.
I'm having issue while playing back video recorder with my phone ( Mi 11 ultra ).
I've also tried every player: from windows stock, VLC and also installed K-lite codec pack with MPC.
Using VLC with HW decode OFF "solves" the problem
I have a thread going at Reddit with users that had issues with this and prior generations: (7) XPS 9520 With Laggy 4K Video : DellXPS (reddit.com).
I also did a ton of troubleshooting, including monitoring clocks and iGPU utilization stats using HWinfo. I suspect you have the same problem (can follow the thread to see) since the i5 version I believe only has the iGPU, and deactivating HW acceleration means you are forcing the computer to use the CPU and since the issue lies with the iGPU, then that "fixes" the issue.
I also compared against a Lenovo Yoga 9 (with the i7-1185G7) and that one also shows much lower utilizations.
I tentatively have a solution - reinstalling the Dynamic Tuning Driver and the Innovation Platform Framework driver, working OK so far 16hrs straight so far. Will update Reddit posting if I find it sticks beyond the 24 hr point.
I've updated the Dynamic Tuning Driver with no luck at all, will try to reinstall both of them and keep looking you reddit post.
Luckly this is my work's laptop and I don't look many video, but I would have returned the pc instantly if was mine, it's unacceptable for this tier
I think this confirms that there is a problem with the iGPU (or a software problem involving the iGPU). I can clearly see that, when the problem exists, the iGPU fails to boost and the Vid Decode and Vid Processing cores are running near 100% (see me thread here: (7) XPS 9520 With Laggy 4K Video : DellXPS (reddit.com).
I spoke to Dell support, they suggested a full system wipe and restore. I did that, and the problem still comes back. Reboots fix it for a few hours to a day or so, but it always comes back. Next step is to get a hardware replacement, hopefully I get this replaced with a new unit given it is so new.
For me the problem still persisted after leaving the computer on for a day or so. It seems the issue primarily happens after the computer stays on for a given period of time, so if you reboot daily or constantly move the laptop around, you may not see it.
I did a wipe and clean install, and still had the issue. Installing Dell Power manager (not the My Dell app) appears to help so far, but in the end I opted for a replacement from Dell given the laptop was so new and since My Dell is supposed to be the replacement for Dell Power manager, so using the latter app is probably a temporary fix anyway).
I have/had this same problem too. 9520 with i9. I've only had it for about three weeks now. The problem showed up randomly after my first two days of use. 4k video seemed to played at 0.5 speed although audio was at regular speed. Then I noticed that ALL video seemed to lag, even 480i. I tried LOTS of solutions, and what finally worked was to uninstall the display driver, and restart the computer. Problem solved... for a while. After a couple more weeks of usage, the problem reappeared. A simple uninstall of the driver and restart of the computer solved it again, but it seems like there's a bigger issue going on here.