Hey,
a few days ago i’ve noticed the very pixelated thumbnails of render materials. This didn’t bother me so far, but i recognized the same effect in raytraced display mode.
I changed cycles from Optiplex to sth else, the result is the same.
maybe it’s related to 4k screens and the posibility to scale windows apps?
I tried to change it, but had no luck.
Check in Tools > Options > Cycles that the performance setting for sharpness is all the way to the right. This will give you slower rendering, but at 1:1 pixel setting.