I get the design decision, but it absolutely is possible to work around it, many games do. Lip sync would get lost, and as a designer that probably bugs you, but as one of the people who likes instant speed text, that doesn't bother people like me.It's a sort of design decision, the problem is that if I allow instant speed, that will break a shit load of things. Look at Pheonix Wright, for instance, you cannot have instant text. The reason is somewhat similar there since the characters have lip animation and that every message box conveys a timed sound (sometimes it can be coming from an environment or something happening on screen), the moment you allow instant text you almost break all the work that went into it. I also don't think it's that slow, well I will upgrade the speed a tad more since there is room for it in the next version, but I might never leave the option to go instant since that will break many things.
As for timed sound effects, there's actually lots of ways to do it in Renpy. I'm not sure how you've coded it, but the most common solution is to have segments of the text load in blocks, cue the effect, then load the next one. Or just do what Dan Salvato did, and lock specific lines of dialogue to playing at slower speeds for the effect.
I like your other work a lot, so understand I don't mean any disrespect. I'm just one of those people who reads really fast, and slow speeds drive me up the wall.