I've been using an X-T3 for years... Now I just decided to give an X-T50 a try, mainly for IBIS and supposedly better autofocus... As light goes down it is much worst, I started to notice it immediately! I tried with bot the 27mm 2.8 and the 35 1.4, I know not the best lenses for autofocus, but still the X-T3 handles it much quicker in difficult/dark situations,
Here is a video, the first is the X-T3, the second the X-T50: https://www.dropbox.com/scl/fi/14mxqd5xkltwz609x3s52/test-focus.mp4?rlkey=xsc6v5zzke2n4m9q4x2wldoci&dl=0
Tracking is even worst!!! : https://www.dropbox.com/scl/fi/9kejtzmlkidyauymh53nk/test-focus2.mp4?rlkey=axg3j51c4nknwgz4ewf9obuj2&dl=0
Both camera same exact settings, both on latest firmware and both in boost mode