[quote]I just had a look at the CVS and it seems that enabling magic URLs does what I said above - puts the message through make_clickable()[/quote]
Not quite, no (note the // before that call)... one of the major things levelled against phpBB 2.0.x was it's comparitive "slowness" compared to other boards. Some people have tried to blame all manner of things for that (usually wrongly

). The major cause of speed differences is simply that 2.0.x did pretty much everything dynamically; bbcode, urls, smilies, word censors, templates ... that takes time and processing power.
In 2.2 what we've (when I say "we've" I mean primarily myself and Ashe

) decided (for now) is to do many more things statically, leaving dynamic processing only where it's appropriate and/or necessary. This means things like urls, most basic bbcode, smilies will be pre-processed. It does "add" more data to the DB and a little extra processing when posting/editing but it means less work at runtime for viewtopic (and reduced load on the server) ... and what's done more often? viewing a topic or editing/posting? ...
The "magic urls" (I thought we always called automatically generated urls "magic urls"?) is done as part of the new message parser. As noted it shortens long URLs (as per requests) and turns urls pointing at the "current board" into relative urls (so if you change the boards location they'll still point to the correct page).