Maybe I’m wrong but that sounds correct to me if you have a more powerful computer than I was testing on (mine using 57% cpu). The timestretch is just running in one process is my understanding, so can’t be split across cores, so the 17% not changing sounds normal to me. But someone more expert might want to comment. The good thing is it’s much faster in both our cases, on slow computer with a couple cores (mine) and fast computer with more cores (yours).