It's not about mining faster in real time. It's about cumulative work. If the BIP110 chain splits at block N and miners switch to it at block N+10000, they don't need to mine 10000 blocks faster. They just need to keep mining the fork chain until its total work exceeds the main chain from block N forward. That's the whole point of longest chain rules. The deeper into the split you go before miners consolidate, the worse the reorg gets.