The current frameskip was designed for FAME core so may not work with UAE core. Thanks for the bugreport.

OSDs take time to draw, so it's normal that turning them off improves performance.

Sound emulation is very dependant on cycle-exact processor emulation, so it's sensitive to performance changes. Amiga sound "system" consists of two D/A converters which data you need to capture and feed to Alsa as a complete buffer to play.