To follow up on #2427 (comment) :

Yes, it will once, but then it just spits out the precomputed fold level for each line till the next change.
Whereas I am under the impression that as Vim implements &foldmethod=expr the function is called for every line and then each such has to redo the computation.
In contrast, this recomputation is only done once, how knows for which line (likely the first), and then stored and retrieved in all subsequent function calls (for higher line numbers).

This might be too apparent an idea, but somehow I never see this caching