A large library should not pose much pf a problem to a P3 at 550MHz. Not for music playback nor database access from a connected Squeezebox.

Where you will notice slow performance (that's relative) is in the web interface. I'm one of the people talking about slow web server performance in the Slim forums at this moment. I even started a thread called "Slimserver wants a fast machine"

There are people running pretty large databases on NAS devices using imbedded processors which are dead slow compared to any P3.

I have about 12000 tracks right now and have used slimserver on a G4 dual 533MHz and currently on a Mac mini Core Duo 1.6GHz.