THIS. experienced the same. got stuck at level 48 then i closed the game and came back later. speed boost came back after i got over level 50.Hmm it looks like the speed boost doesn't activate if your "soft locked" in stages. With that I mean using me as an example, I resurrected to Berserker, did stages and got to a point where I needed to be level 15 to continue the stages. Closed game, came back and noticed the speed boost didn't reactivate until I hit level 15 and was able to progress in the stages again.