I understand what you mean and I agree.
I think in the case of PSX, a lot of the slow downs aren't specific to the PlayBook. I could be wrong, but it has a lot of games that work very well. I don't plan to dig much into it in terms of game compatibility besides merging changes possibly. I'd like to change the UI to cascades for when BB10 comes and add controller support and such when available.
For N64, I've done some quick poking around to see why it might be so slow for games like Zelda. It's nothing higher level like the input overlay, or audio. I'm reaching out to the touchpad dev to see how it runs on the TP to rule out Mupen specific issues. For me, Zelda OoT is almost playable with the sound off. (You've tried the (E) version on glesn64?) There is a LOT of code for these, and it takes a big commitment to really dig in and fix something. I don't have the experience to write something like this from scratch in any reasonable amount of time, but I can at least track down and fix issues. The next steps would be to update the Mupen specific code, or try and profile the app to see where the bottleneck might be. I'd also like to add a cascades UI to it.
For both projects, I've been super busy and haven't had the time or energy to devote to tracking down these issues. My full-time job is also dev work, so I'm already programming 8+ hours a day.
I will definitely try to fix these issues if no one else gets to it first. It just may take some time and patience.
In other news, I'll have a fix soon to toggle the Z button from the left side to the right.
EDIT: I've been told that the start of Zelda in the forest is one of the worst performing parts (~10fps with audio stutter) but it should be better in later parts. I'll try and test this if I get some time.