First thing to check is to Open Manage | External Data Sources on all yoru files and check for issues.
Files that started out as much older versions such as version 5 can have very complicated lists of file references for each related file. If you have a lot of spurious file refrences, such as an IP address that no longer exists, FileMaker can take a very long time working through the list of file references until it finds one that opens the file.
Likewise, references that contain an asterisk, *, often used in older systems, but resulting in very slow file opens in the later versions, should be removed or updated so that you no longer have them in the system.
There are other possibilities, but given the history of your files, I'd check this first.