I find updating tags slow on my system as well. Definitely faster on the local machine than via the mapped network drives. But, I'm with Rolf, I don't really know why this has to be. I do not have any experience of slowdown with other files (for instance, photo files accessed, edited and managed via Adobe Photoshop or Album v.2). Playing music files in MC is not noticeably slower. But tagging definitely is. Now that tagging happens in the background, I don't notice it as much (before it went into the background, I would practically cry with frustration 'cause my whole system would be locked). When I go to close MC, though, it is almost always still tagging files. . .
This problem definitely started with MC11 (maybe somewhat with 10, but it wasn't so noticeable for me until 11).
Just wanted to chime in.
BTW, my system is a Linksys Network device that had 2 Maxtor HDDs on it.