1541 with demo's fastloaders

Hi all,
I have recently repaired a few 1541s (various revisions, various
mechanics) and I've found a quite strange situation with one VC-1541
from 1983. It has a short board with quite some wire patches on the
solder side (they look quite well made, so I think it's as it come out
from the manufacturer). This unit was modified with the speeddos
parallel cable and I removed the mod since I didn't need it. The
mechanics was still almost perfect, it needed just a good cleaning,
after an easy electronic fix, the 1541 was reading and writing quite
well.
I usually test a few demos that use quite optimized fastloaders to
make sure all is well with the drive before declaring it "good".
My favourite demos are Lunatico from LFT, which uses a very clever
fastloader, it's described in his blog, I really recommend reading how
it works as it uses also 6502's unofficial opcodes and other clever
tricks.
However, this particular drive worked well with the LFT's demo.
Then I've tried the Concert demo from Performers and the drive showed
a quite an unexpected behavior: it seemed slow like the fastloader
didn't work, but the demo progressed into the third disk anyway and
just before the concert part (if anyone is familiar with this demo, it
hangs after loading the "rock me amadeus" cover part. I'm sure the
disk is good and anyway this 1541 reads the disk fine when I try to
copy it.
Now I suspect some tricks in the Concert demo hit a particular hidden
fault of this drive.
I'll try a different 6522 on UC3, most old speeddos mods tend to
damage that chip.

So the question: does anyone know how this fastloader work?
I'd rather not disassemble a whole demo :)

Thanks
Frank