Same instructions from the other link posted, different source: https://forum.zorin.com/t/speeding-up-steam-shader-processing/50008

1. Navigate to ~/.steam/steam (This should be a symlink to wherever your Steam install is located). If the folder has a steam.sh then it is the correct folder.
2. Make a file called steam_dev.cfg
3. In that file put: unShaderBackgroundProcessingThreads x
4. Replace X with the number of threads to use. (See below.)
5. Save the file and restart Steam if it was running.

For X, modern processors generally support two threads a core. If you’re using a 4 core processor, you could have 8 simultaneous threads. if you’re using an 8 core, 16, etc. You don’t want to use that number for X though, or Steam may hog your CPU enough to make your system sluggish. The original poster recommends your max threads, minus 4 to 6. I’m using 10 out of 16 threads, personally.

As the Reddit thread notes, despite the name of the setting, it does work for foreground processing as well as background. One thing the thread doesn’t warn about is that if you keep Steam open and a game updates, Steam may start working on these in the background, if you have background shader processing turned on. If you’re doing CPU intensive work, you may want to close Steam or turn off background shader processing to keep Steam from monopolizing your CPU.