Time-domain pitch-synchronous overlap-and-add is a remarkably simple but effective way to independently modify the duration and F0 of speech.
Start with this post, that shows how TD-PSOLA works in practice
then move on to the reading
Reading
Holmes & Holmes – Chapter 5 – Message synthesis from stored human speech components
Pitch-synchronous overlap-and-add (PSOLA) remains a key technique in speech signal processing.