time between waves is good, think about it:
to play in hard you must sell for 100% and build the specific tower with the specific attack for the next wave, and upgrade it, and change between rocks, glaciers and poker temples... you need time to do this...
plus, I've started this map with 1:50 between waves, and THAT was long, 50 secods is allright... I will put 40 seconds in next update for you to test it ^^