ayoob,
I just tagged a mixture of 388 FLAC and MP3 files in 12 seconds on an older PC (Athlon XP 3200+, a local, almost full 300 GB sata drive). The files contained padding area that reserves some empty space for tag changes so there was no need to rewrite the files even though these file formats store tags in the beginning of the files (Vorbis/FLAC comments and ID3v2).
By default MC includes some padding when the files are created or tagged for the first time. Small textual tag changes should be almost immediate. A worst case scenario could be embedding high resolution cover art to big FLAC files through a slowish old wireless LAN. The padding area is only a a few kilobytes so a complete rewrite would be needed. For instance, if the file size is 50 MB and the actual practically achievable network bandwidth is about 5 mbps (= 625 KB/s) tagging would probably take about 3 minutes (read 80 s + write 80 s + some additional time for local file operations, network congestion, etc).