By the time 60 or 90 minutes has gone by since the last block, the mempool would be so large with fees that 100% of hashpower should be online trying to get them
Conversely, after a block is mined the mempool should be empty and without a subsidy it may not be profitable to mine until mempool reaches a certain size.
So there will be pressure for low hash rate before 10 minutes and high hash rate after 10 minutes, giving a much more consistent 10 minute block time.
