I feel the creeps take very long to kill in the early game. I guess this is to prevent you from one-shooting the same creeps lategame.
How about having week creaps in the first spawn, and then let them become stronger on each spawn. Not indefinitely, just 1-3 times (until players have reached their t3 armies)?
The rewards could also increase, adding some ramp up to the game (titans and heroes and uppgrades ramp up, so it would be logical for creeps to do the same).