I removed that functionality at some part of the UI development. It's available on Android but not on Desktop as it's an option I do not use myself...
I've made a quick auto toggle button if you want to use it, you only need to drop this file inside Lost at Birth/game :
https://www.mediafire.com/file/fy6ymmf4fvb5ojw/auto_toggle.rpyc/file
To control the speed of the auto dialogue use the "Auto-Forward Time" slider in the options menu. The lower (to the left) you move it, the faster the auto scroll will be.
