Stuttering and frame drops when watching 4K videos (windowed only)

[H-Koboshi]

Specs

Computer Type: Desktop
GPU: Sapphire PULSE AMD Radeon RX 7800 XT
CPU: AMD Ryzen 7 3700X
Motherboard: ASRock X570 Steel Legend WiFi ax
BIOS Version: P4.10
RAM: 32 GB Kingston (3000 MHz)
PSU: Cooler Master MWE 750
Case: Cougar MX330-G
Operating System & Version: Windows 11 24H2 (26100.3476)
GPU Drivers: Adrenalin 25.3.2
Chipset Drivers: AMD Chipset Software 7.02.13.148
Monitor: ASUS VG248QZ (1920 × 1080, 144 Hz, no FreeSync or similar features) (no other monitors plugged in)
Background Applications: N/A (issue is present with and without background applications)

Issue

I recently “upgraded” from Windows 10 to Windows 11 (via new clean install, nothing left behind), and I've started to notice videos (for example, 4K in YouTube, but I've seen it sometimes in local video players) stuttering and dropping frames when watching in windowed mode. The frame drops stop when entering full-screen mode. I've also seen some black flickering on stuff like Discord group video-calls, but it's not frequent or easy to reproduce. The frame drops can also happen when watching streams on Discord, and they happened with MPC-BE (had to mess around in video decoding settings to fix it, suggesting there's something off with either DXVA and/or DX11 for video acceleration). This didn't happen with Windows 10.

EDIT: I cleanly installed Windows 11 with the same media on my Intel+NVIDIA laptop (which is way weaker), and this doesn't happen.

What I've tried

• Disabling MPO with this. No difference at all. It seems 24H2 doesn't allow MPO to be disabled in the same way as in previous Windows versions. DxDiag still reports MPO features after running the disabler and rebooting.
• Disabling ULPS. Didn't seem to be relevant, but tried it nevertheless. No changes.
• Checking variable refresh rate. Not applicable, my monitor is not compatible.
• Checking GameDVR. The “Record what happened” stuff is already disabled.
• Messing around with Firefox about:config switches. Not relevant anyway, since I tried other browsers and the problem stands.
• Trying different browsers. The stutters happen with Firefox, Edge and Chrome, in the exact same way (happen in windowed mode, disappear in full-screen).
• Trying to set/unset hardware-accelerated graphics scheduling. Not available for the RX 7800 XT.
• Disabling windowed optimizations for Firefox.
• Checking “Disable fullscreen optimizations” for firefox.exe.
• Updated the UEFI/BIOS to the latest version.
• Enabled the TPM (I had disabled it).
• Downgrading to Adrenalin drivers 24.10.1.
• Changing screen bit depth to 6-bit (the default is 8-bit).
• Enabling/disabling resizable BAR.
• Enabling/disabling video enhancements in Adrenalin and in Windows Settings.
• Forcing V-SYNC off in Adrenalin.
• Forcing Radeon™ Enhanced Sync in Adrenalin.
• Changing display topology with CRU (info here).
• Enabling MSI for all devices, and using High interrupt priority for the GPU.
• Using the AMD Ryzen™ High Performance power plan.
• Using the bcdedit /set disabledynamictick yes command.

Clues?

• In Firefox, I checked its internal Task Manager while a 4K video is playing. There's a button to profile each process' thread for 5 seconds. I did that with the “GPU” process, and the video stopped stuttering afterwards (until I close and re-open Firefox). I am not familiar with this, but these two entries in the tracing might be interesting:
• 
• When using MPC-BE, which comes with MPC Video Renderer, the same stuttering can be seen with any GPU-accelerated video. The stutters stop when the “Use Direct3D 11” checkbox is unchecked in MPC Video Renderer settings. The playback then falls down to DirectX 9.