If I read this correctly, the file resides on a fileserver. Then the loss of performance might have nothing to do with the file itself but with memory/caching problems. Can you exclude that there are too many(big) files open, so that VFP/Windows starts using virtual memory?
Does the server have enough memory per client for caching?
Can you try to open the files in your developers environment one after the other and find out WHICH file first loads slowly - and if other files after that load slowly too?

Regards