Is this assumption correct?
Yes. That's right. If you choose a render speed that is < 100%, every time based function in Isadora is slowed down proporionally (i.e., Wave Generators, Envelope Generators, Trigger Delay, et. al.) This gives Isadora more time to generate every frame and thus should result in full and accurate frame rates. The only caveat is that you cannot have real-time input because it is not slowed down proporionally.