I don't really know why your Macs are loading quicker than the PC's, but there are a few things you can check--if only to rule them out as possible factors.
Check for any scripts that perform when the file is opened. If you have such a script, remove the reference to it in File Options as a test and see if the windows machines load faster. If they do, look the script over for any possible causes.
Open manage | External Data sources and examine them for issues.
External file references that use asterisks and external file references that contain invalid IP addresses can really slow down the rate at which a file opens.
There are no external data sources that are used.
I will try your script tip.
The biggest difference between the Macintosh computers and PCs is in the one layout that lists all of the projects. The list is 144 records long, as I mentioned above. On the Macintosh computers you can scroll up and down the list and the screen and there is no delay or refresh. However, if you perform this same exact thing on a PC, you have to wait for the list to refresh if you scroll up or down. Very odd and frustrating for PC users.