It's understandable that reading things takes time. The strange thing is
that this should all be cached, so second runs are much quicker. Can you
check whether the font directory are changed (files (dis)appearing,
timestamps that change?) regularly?
Also, there should be a font.cache-1 file, which caches the results. I'm
not sure where it is put on Windows, though.
Hmm, I can't find anything called font.cache-1 or even font.cache*
anywhere on my computer. Perhaps this file is not created on Windows
anymore? Lilypond has been slow since 2.7.12 I think. I may try
installing an older version and see if the speed comes back.