SUMMARY

My machine (POWER9 ppc64le, DD2.3 stepping) has two GPU's: an integrated ASPEED AST2500 (ast Linux driver), and an external SIIG JU-DV0112-S1 (DisplayLink DL-195 chipset, udl Linux driver, connected via USB). Both are unaccelerated, so 3D operations use LLVMpipe. The AST2500 seems to perform fine with Plasma (no perceptible lag), but the DL-195 exhibits nontrivial video lag (~1-2 seconds) with Plasma. Both GPU's perform fine (no perceptible lag) with GNOME Classic, so I'm reasonably confident that this is not a hardware or kernel issue.

STEPS TO REPRODUCE

1. Plug in a DL-195 USB GPU.
2. Configure Plasma to use the DL-195 GPU to extend the desktop to the right.
3. Drag a large window between the main display and the DL-195 display, back and forth, as fast as feasible with the mouse, for around 15 seconds.
4. Release the window while it's on the main display.

OBSERVED RESULT

The main display will show the window no longer moving immediately, but the DL-195 display will continue to show the window moving back and forth for around 1-2 more seconds, i.e. the video is lagging on the DL-195 display.

EXPECTED RESULT

Both displays should show the window no longer moving immediately, i.e. no perceptible lag.

SOFTWARE/OS VERSIONS

Operating System: Fedora Linux 38
KDE Plasma Version: 5.27.4
KDE Frameworks Version: 5.104.0
Qt Version: 5.15.8
Kernel Version: 6.3.0-0.rc5.20230407gitf2afccfefe7b.46.fc39.ppc64le (64-bit)
Graphics Platform: Wayland
Processors: 144 × POWER9, altivec supported
Memory: 14.8 GiB of RAM
Graphics Processor: llvmpipe
Product Name: T2P9S01 REV 1.01

ADDITIONAL INFORMATION

Happy to provide more details on request.