EDIT: Rob beat me to it
Did you read my reply where I mentioned what I thought the problem is with the file? (the repeat structure - specifically the missing start repeat barlines).
Dorico can make use of multiple cores, but only for certain operations that can be done in parallel. If you load a large orchestral score and watch the CPU meter then you’ll see all cores in use during parts of the loading process. However, there’s also a lot of the processing which has to be serial, and in this particular case, the (incorrect) repeat structure takes a very long time to calculate. I would suggest turning off the playback of repeats, fix the barlines and then turn playback back on again.