I’ve changed this to 1080p in the launch menu and now all games are running perfectly with shaders on!
Can anyone recommend an optimum resolution? Interested to see if any of the others may make further improvements.
1080p60 is the optimum really. any lower may increase performance if gpu is the bottleneck, but if it's already full speed it's not going to matter.
also, the crt scanline shaders require ~4 vertical pixels per emulated pixel for the scanlines to look decent, and 240x4=960, so 1080 is near the typical limit anyway.
finally, lower resolutions = more obvious scaling artefacts.
perhaps some of this info should be added to https://retropie.org.uk/docs/Shaders-and-Smoothing/ - will try and do this at some point today!