I deal with the Instawallet shared wallet, which is probably an order of magnitude larger than Intersango's, and yes, it takes a little while to boot up, but when it's done it runs fine and smooth.

Now that I think about it, it might be that they're using the 'accounts' feature of bitcoind, which scales terribly. So guys, protip, use a monitortx patch and let bitcoind notify your backend, just poll the client to update confirmation counts once it notifies you of an incoming block.