Trust me, 90% of the site content is in memcached.  Remember GLBSE?  That's what a site like this is like without memcached.   

You cannot use memcached for authoritative data though.  IE, you can't use it anywhere in the trade process or with trade data because you have to be prepared for the possibility of server crash or memcached crash or memcached purge at any time.

The biggest slowdown right now is that we're double-tracking account balances while we try to implement new balance tracking.  We started a year ago using bitcoind to track accounts and balances, but as the db grew, it got really slow, so we're transitioning now to using an SQL server for the account balances.  That transition has been slow, as we've been trying to be extremely careful to make certain that the account balances match up between the two systems.  (and occasionally, due to things we're trying to work out, they do not...)

So the bitcoind wallet balances are still considered the authority, and we're working out the kinks in the new system so that we can make the SQL server the authority and retire bitcoind.

I hope that makes sense. 

Cheers.