It looks like that's just a minor graphical issue with that version of PocketSNES, the one based on version 1.43+SDD1.
I checked it on my PocketGo 2 with an August 2019 build, and on my GCW Zero with an August 2014 build, same issue and performance.
I checked a build of PocketSNES based on version 1.60, and it doesn't have this graphical issue, but the performance is much worse, presumably due to needing to emulate more accurately.