Wouldn't it be possible to disable filter distortion and other expensive operation while seeking that are not necessary needed while seeking?
I haven't looked into this deeply but I think you would loose accuracy once the processing returns to normal playback. Anyways, residfp removed all accuracy sacrificing optimization options, so this would require quite significant changes to the library.
BASIC sids haven't 'Speed' in general info window.
Actually RSID songs should have no speed displayed at all. CIA is wrong. I'll correct that in the next update.