A thought on that; instead of making the wave faster overall, why not a mechanic where the longer an enemy has been alive, the faster they get? An acceleration value, in other words. You can make it higher for the higher-tier enemies (so the reds would have a higher value than the blues, for example). That would make it still a safe wave early, but if you faff about too much without addressing the enemy, they can quickly overwhelm you, too.