Hey @grub some answers here: Commit merging on pull and online viewer - #2 by dimitrie
Speckle is currently only borrowing git terminology, aaand it is actually going away soon 
Diffing is done behind the scenes for you, so a Revit element that was sent in a previous commit is not sent in future ones, unless it has changed.
In regards to performance, there is still additional optimization that we’re planning on doing, in some connectors (eg Revit) we have to convert and serialize each object locally before the diffing happens - but we will soon be able to do it before conversion and make things even faster.