Thanks for the file. There seem to be a problem with the features.fea file and the macOS regular expression engine. I replaced it and found some more place that I could improve performance.
Curious, after the repair, UFO import will be how fast?
15 s on my MacBook Pro with M1 Pro. Opening the same data as a .glyphspackage takes 8.5 s.
Amazing ! hundreds of megabytes of files .