I was playing a 100 player game over the weekend and passed some saves to Brad and the dev team to check out (to investigate perf time). The got the time between turns down from 92 seconds to 30 seconds the last time we measured. We continue to make adjustments, so those stats should continue to improve. You will see a list of perf improvements in patch 3 for exactly this reason.
Most of these perf improvements make the game faster and smoother even on normal small maps, so its a win even if you aren't playing at a ridiculous scale.