17:49
@eponymous - it's problematic when there are more than approximately 5 transactions per second, regardless of the actual fee paid by each transaction, since the capacity of the network is only around 5 transactions per second regardless of fee.
i.e. it's possible that some transactions will never get processed if there are consistently over 5 transactions per second happening, regardless of how high the fee is. I can imagine a state where a transaction might need to pay a fee of 50% of the value of the transaction just to have that transaction be processed at all. Of course, it's arguable that the network will have failed long before that.
and the mempool number I was quoting is based on a combination of observation of my own node, and looking at the number of outstanding transactions via blockchain.info
clearly if your node is not dropping "old" transactions, it will have more outstanding transactions in its own mempool.