Speed up seems to work somewhat incorrectly:
When just starting building up the latest levels I see that monsters pass to my gems most of the time when I hold the speed up while exactly the same build of towers successfully defend from the same waves of monsters most of the time in the normal mode (without speed up).
 "Diggy"){kind=icon}
 "Caribbean Admiral"){kind=icon}
 "Ultimate Tank War"){kind=icon}
 "Frantic Sky"){kind=icon}
 "Cursed Treasure 2"){kind=icon}