Sorry, but you really have no idea what you are talking about so just stop giving advice. 300mb IS big for a quickbooks database.
Quickbooks is a PIG, especially when it comes to searching on large database files. Are you using the SQL based version of enterprise? It is significantly faster than all of the flat file based versions.
Have you tried switching all of the clients into single user mode and then adding back one client in multi-user mode and seeing if that client performs better? Quickbooks has a weird problem when multiple clients are even associated with the multi-user server and can cause it to bog down completely.
Personally I have moved many companies OFF of quickbooks due to these very same issues. It is an inferior product performance wise once you get over 1-2 users, regardless of their claims about it being able to support up to however many users.
One other thing you can try is to re-load an old copy of the database from before you started seeing performance issues and see if it works fine with the current system. I would bet it works fine and that the main issue is that quickbooks simply blows once you have a certain number of transactions and objects in the database file. It isn't a hard set limit, seems to vary depending on how your customers and fields are associated.--
Broadline Networks Inc.