Yes, that’s hard to avoid. When you are raising the pitch or speeding up the tempo, you are feeding the stretcher more and more samples that it needs to crunch on. It needs to process a lot of input for a relatively small amount of output.
This could only be optimized by ignoring/skipping parts of the inputs, but this is tricky to realize - cause you then somehow need to decide what to ignore and what not to retain the original character of the sound.
That said, this also is a bit out of my control. We are only using the library. The author and thus the only one who could change or improve that is Chris from breakfastquay
I could collect issues and wishes though and send them over in a compact and moderated form.