MultiBit Classic is a bit slow when syncing lots of open wallets, or a wallet with lots of addresses in (>500 or so).
There's not much you can do about it other than make sure your network speed is fast enough.

Also check you are on the latest version and upgrade if you are not.

If you are finding it just to slow to use then a good thing to do is actually move the wallet/ private keys to blockchain.info (and then your using their servers effectively.) There is a help article on how to do this in the MultiBit Classic help:
https://multibit.org/en/help/v0.5/help_movingAWallet.html