There are a number of changes there.
The FileMaker file has gone through a conversation from the .fp7 format to the .fmp12 format among other changes.
I'd be looking at the solution and seeing what has changed there during the .fp7 to .fmp12 conversion.
Second I would look at the changes in the network/file storage.
The fact that it is Windows or Mac shouldn't make a difference, if you have access to a Mac machine I'd assume that you would see the same slow performance on your new filemaker files.