I was able to try Retroplayer on an entirely different machine/network/display/controller: Intel i5 2500k, Geforce GTX 1060, Windows 10, brand new Kodi 19 install. Tried every available SNES core, all had the same exact problem with FPS drops as described in the OP.
What am I to make of this, if apparently nobody else can reproduce it? I'm yet to see any working Retroplayer configuration. I suppose I have definitively confirmed this is not a LibreELEC issue and may take it to Github, but hard to say when nobody else is complaining about it.