I'm wondering if when the file was on the 64 bit machine, the user there didn't leave a lot of sections/elevations, schedules, layouts, etc windows open before saving - and they are all rebuilding when you open the file now. Try closing all windows other than the floor plan (ctrl-shift-w), save the file and re-open. Does it still take 15 minutes to open? Are any libraries on the network, so that it is a network issue rather than an AC issue?
Cheers,
Karl
AC 28 USA and earlier • macOS Sequoia 15.3, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB
One of the forum moderators