This is an interactive game, where you press the magnet to break of the ball stream into better paths, or keep the balls in a pocket of zombies.
When I realised this, I no longer needed a speed up button, nor did it bother me that the bounce was odd sometimes, actually it only meant more opportunities to get some balls where I wanted them. (Strange bounce only bother me now when I have a superball)
So if you want to have speed up button, I would say you are not playing the optimal way.
Nice game. I had some bugs/flaws while playing, though.
1) I cannot use a potion if I am at max health. Often later in the game I used potion for the stamina, not the health.
2) If I have more than 100 points to spend, the number grows outside its box.
3) When I used/clicked the light orb? I had the buy option open and it was gone when I continued to the rift.
4) The hp indicater was confusing at first at I thought that it was the hp of the monster.
 "Kingdom Rush Frontiers"){kind=icon}
 "Balls vs Zombies"){kind=icon}
 "Monster Clearer"){kind=icon}