Hello mike b,
Welcome to the world of SoundFonts!
TL;DR: Speed and pitch are bound to each other; you can't change one without changing the other. Also, if you do that, the sound quality will be worse.
Unfortunately, in SoundFonts, what you are describing is not possible.
If the pitch or length of the sample is increased, so is the speed or pitch.
This means that it is impossible to simulate this behaviour with SoundFonts. Tweaking the volume envelope or the loop section will not slow down the loop without reducing the pitch, or increase the pitch without increasing the speed.
There is a program called 'Audacity' that can stretch or increase the pitch without speedup, and results can sometimes be very high quality, however in most, if not in all cases you will get a permanent loss of quality. Therefore, it is not advisable to do these processes to samples. If you want the highest quality, you will have to add more samples.
Thanks,
Strix.