Hello, Im using 14" screen, it shows the realdash theme stretched and ugly, so i changed size of mid-sement and gauge arc from 416px/416px to 416px/832px it worked for me.
But when i want to scale speed gauge and rpm gauge to the same value it became black on linux :
Yes, reason I wanted you to try is to verify that problem is on off-square needle gauge rendering.
When your needle gauge is square, there are quite a lot of performance optimizations that can be done for the rendering. If it is off-square, a different (less-performant) rendering path is taken.
Unfortunately we do not have any RPi3:s anymore to test on. I will test on RPi4 & 5 to see if I can reproduce the problem there.