GIMP slow and unrespsonsive when working with AMD CPU

GIMP Version: 2.10.18

Operating System: Windows 10x64 Pro

Package: Gimp.org installer

Issue Description

Scaling or filtering operations that take seconds in RGB can take up to 10 minutes to complete in grayscale (built-in). During this time GIMP is usually marked as "not responding." The slowdown is extremely consistent and repeatable. EDIT: This also happens with RGB, the issue is not limited to grayscale as I thought.

Performance Log Description

I began by scaling a 2048x2048 texture down to 512x512 in RGB, then converted to grayscale and repeated the operation- The difference in completion time between the modes is massive. I can attach the texture used if needed, but the results are the same with any large grayscale image. gimp-performance.log

Additional Information

This seems similar to #2964 (closed), but I created a new issue since this only happened after switching to an AMD CPU (Ryzen 5 3600). My previous Intel CPUs (i5 6600k and 2500k) never suffered from slow performance like this. While downscaling the texture above task manager showed total CPU usage around 42% with all 12 threads in use but none maxed out.