I also discovered that the system is a bit dumb, in fact the so hardly computed hash key could be stored in a file, in order not to compute them again for unchanged file at each restart of LimeWire, but it is not the case! Every time I restart LimeWire it computes all he hash keys again.
I hope this gets improved in the very next future or this will affect the amount of songs everyone can effectively share, since I (and many others) don't stay connected for 8 hours in a row...