If you have a 3 speed, obviously 3rd will be the fastest option. Looks like you have a 2 speed. Probably 2 things to check, belt not getting tight, you can adjust the nut on left side to put more tension on belt, The other is transmission is slipping which would mean a replacement, from what I have researched that transmission is pretty durable. I guess it could be that you are not turning 3000 Rs, idle is 2400 or so.