Me and a partner have an active teamcreate session. At 33,000 parts including several union operations, and a file Size 121 MB, the game had slowed to 30s save times in its original published place file.
Upon downloading as a local copy, and republishing to an entirely new Roblox Website Place, the save time was brought down to 5s for the same alteration to file.
For scientific control, saved a few times and moved a basePart back and forth to change file structure each save for all statistics. Also conducted over a few days and persisted.
Here we see the visually identical file compositon in an old website place
and here we see it in a new one, with reduced timestamps during one of a few trials.
This would seem to suggest there is a “magical factor”, something beyond the scope of the file itself influencing save times.
(Mind you we kept everything the same, but the publishing ID. Yes that means teamcreate, same Group same file same everything. The only difference is PlaceID.)