Every time you import a wallet, it has to check the blockchain to determine if there's any balance.  If you have lots of wallets to import, the best way to do this is to stop Armory, and restart it in offline mode.  Then import all the wallets at once, and restart Armory in regular online mode.  It will rescan, but it will only do it once.  In the near future, we hope to have that rescan process be a bit faster...