Open manage | external datasources and take a look at the references stored here. Look for the use of asterisks or IP addresses that are no longer valid. Either such entry can greatly slow the rate at which your files open. Since you have multiple files, you'll need to check this in all of them.
You may find a lot of duplicate file references and garbage entries cluttering these up as FM 5 didn't do near a nice a job of managing these and then didn't allow developers any access to them to fix them. If you choose to clean this up, proceed carefully making a back up copy before each change just in case removing/changing a reference here breaks a part of your system. (And FileMaker Advanced's database design report can be very useful when cleaning up these references and eliminating duplcate references to the same file.)
where can i find the FileMaker Advanced's database design report?
Pull down the tools menu and select "database design report".