Can you split the library in half and see if one half is good and one is bad?

If so, then split the bad half and add to the good and see what happens. Keep splitting the bad half. With 4000 odd books it would narrow it down within about 12 goes.

I'm not sure a corrupt book file could cause such big delays though.

Could it be reaching out to a non-existent URL that is timing out?
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Christian
#40104192 120Gb (no longer in my E36 M3, won't fit the E46 M3)