They don't seem to matter.
I was transferring Bitcoin when the backlog was up at 220,000. Only paid a tiny fee and got 3 confirmations within 12 minutes.
I think it's just 0-fee spam.
I used blockchain.info last week with default fees and BTC took 72 hours to confirm. Another transaction from coinbase was 3x more but took 15 minutes or so. Looking back on the blockchain.info transaction, might have been made up of a payment from many many inputs.. but either way, it took 3 days.