Tested additional mainline kernels installed via UKUU. 4.19.66 LTS worked great. So did 4.20.17, the last in its series. The performance regression appeared when I tested 5.0.3, and as noted above, still persists in 5.2.8.
This may be a mainline kernel bug, but I'll leave it open here for further input. I'm a complete layman in this area.
Tested additional mainline kernels installed via UKUU. 4.19.66 LTS worked great. So did 4.20.17, the last in its series. The performance regression appeared when I tested 5.0.3, and as noted above, still persists in 5.2.8.
This may be a mainline kernel bug, but I'll leave it open here for further input. I'm a complete layman in this area.