I notice that when I time stretch a track that it seems to mainly focus on one core. The CPU is mainly idle…and the time stretch takes a loooooonnng time.
(windows 10)
why ?
There are various algorithms, some are much faster. Try them, there are good too. The quality is not proportional to the processing time.
But by nature, a time stretch algorithm is mono thread, as this is a sequential process.
So basically not a problem, just wait it out. 