I haven't played with Paulstretch, but it looks similar to what Tom Erbe's "Soundhack" program did (windowed FFT with overlap add resynthesis). Soundhack was originally written in the late 90's and was used by the sound designers of The Matrix.. anyhow. Soundhack is now defunct, but Tom released the source and I ported the time stretching bit of the software to Go if anyone's interested: https://github.com/corporealfunk/gopvoc