This is something that I just ran into, as not many people create songs that go this fast to begin with...
I finished an .xm I have, and I was not happy with the pitch sliding on one of the instruments. When I made the instrument go solo, I heard that the frequencies seemed to "stair step" rather than smoothly glide from the starting frequency to the end frequency. (End of use with the portamento effect) I knew there was a way to smooth it out, speed up the song. Originally, my tracker was running at 150 bpm and a speed of 2.
Here's where the issue arises.
I sped my song up to a speed of 1, keeping the bpm the same, however, when the instrument got the portamento effects, to pitch slide it up and/or down, they were ignored. Now, I have thought of whether it was just Milky tracker screwing with me, or not, but when I exported the .xm and tried it in Open MPT tracker, it had the same issue. So I went to the manual and maybe I'm missing something, but I don't understand why neither of the programs read the portamento effects. Is there a reason for this that I just didn't see, or is it literally "too fast" for the coding in the programs to detect?
If only my instrument didn't have the "stair step" frequency shifting, this wouldn't be a problem.