So I ran vam into a sandbox and traced all file access (950'000 lines to look at..). Rough analysis:
Loading time of VAM independently of vars is strange, small reads of files in VAMdata.
Then obviously VAM parses all vars, prefs, fav, .. but it seems rather fast.
I haven't seen particular VAR creating problem (but I'd need to plot time differences between vars..)
On my particular installs:
Bad files have been rmoved: wrong naming, corrupted ones (broken zip format). I don't think they affect much time as it didn't change much. Also had one with circular dependencies (A requests B and B requests A).
I also removed massive var which include others content.
So I can't tell you which ones, I erased them already.
I also removed all custom old format files (300GB).
Overall it didn't change much.
In the end, for reference, I have 120GB within 2500 vars.
Windows: Stupid defender keps on scanning a 120KB file which is from.. Microsoft. Also search indexing tries to index content of var (I associated .var with 7Zip, I wouldn't advice doing that) and also pdf (I have a few hundreds and some are 40MB+).
So... at first glance, it just seems to be the amount of vars that influence loading time.