It depends on the game, I guess. In some games, the text speed may be managed by plugins.
In a generic Japanese game, the text showed instantly when I opened the file
www\js\rpg_windows.js
in a text editor, found the lines:
Code:
Window_Message.prototype.clearFlags = function() {
this._showFast = false;
replaced
false
with
true
, saved and relaunched the game.