From Ren'Py

Default text speed Setting

To set the default speed of text, open options.rpy, and change the line that reads:

    config.default_text_cps = 0

Replace 0 with the number of characters that should be shown per second. Valid numbers are in the range 1-100, with 0 being special-cased as infinite speed. (Note that this is somewhat odd... 0 is infinitely fast, 1 is very slow, and 100 is very fast.)

To make this setting take effect, close Ren'Py, delete game/saves/persistent, and then re-launch your game.