Events 2 and 3 in your screenshot run on every tick while their conditions are true. You should avoid using "Wait" in events that run on every tick, because this creates hundreds of delayed threads, which continue running for 1.5 seconds even after the condition becomes false, messing everything up..
Use Timer behavior instead.
On collision with the boost : Increase max speed, Start Timer "reset_speed" for 1.5s
On Timer "reset_speed" : Set default max speed