The way to do it is only on the browser to use plugin. Video Speed Controller - Chrome Web Store
The tip was stated before buried in the middle of a post and many seem to ask for it which plex should have just made some suggestions if it does not know how to program or design well. Even the similar topic suggestion gives the questions for people to ask for it.
Speed is good for boring parts, impatient people, to put sleep down to read subs, or slow down to see actions parts better at a click of a button. I usually use but plex reserved it so its a guessing game where one should put it. Any suggestions? 
