Transitions that use volumetric fog are the expensive ones — the fog is sampled per pixel per frame. Frames outside the transition render almost instantly, so a 129-frame clip is not 129 slow frames. If you need it faster, lower the volumetric sample count, or turn the fog off in Scene FX for transitions that do not depend on it.

Yes. Everything in Preset Settings edits your scene, not the preset itself. Reset puts the transition back to how it ships. Note that the scene keeps its own copy of every setting, so if you want a preset's shipped values back after editing, press Reset rather than reselecting the preset.

Everything it makes goes into a collection called TransitionFX and is named with a TFX_ prefix. Your own objects get material and visibility animation but are never moved, renamed or replaced. Clear Transition removes the whole collection and strips the animation it added, leaving your models exactly as they were.

Any mesh. The add-on measures the bounding box of whatever you assign and sizes the effect to it, so a coffee cup and a spaceship both work. If the effect looks too large or too small around your model, adjust Scale.