I have noticed that when using au or vst Fission in Ableton, every instance of Fission that is added adds more latency to Ableton. This is not acceptable for me as it adds latency to midi input on tracks that are not even using Fission. With this it is barely worth the discountd price. I do like the Bowed Guitar for removing beats though.
Fission uses a complex process for separating transient and tonal audio, which takes a noticeable amount of time (latency). Many plug-ins do this, to some degree, and provide the amount of latency in samples to the DAW, so that the DAW can offset this latency (i.e. move other tracks back by the appropriate amount of latency) upon playback.
If you are recording something or have the Monitoring on in Live, then you will notice this latency, because Live can't shift tracks appropriately. When tracking MIDI, you should bypass Fission, or render tracks using it to audio.