From what I can tell, text speed is set individually for every line of text, so changing it is a huge fucking pain.
Part of the issue may be text crawls one character at a time (I think), and since Japanese uses fewer characters per sentence than English (probably), it winds up being much slower.
Pressing enter (or whatever your confirm key is bound to) will instantly finish the current line of text, which is useful if things are taking too long. Other than that, nothing is particularly usable, sorry.