Because of mining fees and the way that miners give priority to transactions based on the fees they recieve we can no longer tell how long it will take for a transaction to occur.
In the past it would have been possible to calculate the time before a uniform transaction wait period occured by seeing how many transactions were in queue and how fast mining was occuring.
But due to miner's preference for transactions with variable mining fees some transactions are now sitting indefinitely in the mempool.
We can no longer accurately tell how long, or even if, a bitcoin transaction will take place.
This worry of "what if my transaction fee is too low and won't be processed" means instablity for bitcoin as a whole and causes a problematic dynamic for cryptocurrency exchanges as a whole due to volatility.
Imagine that you are exchanging cryptocurrency in a volitile market and you don't know when your bitcoin exchange will take place between yourself and your cryptocurrency exchange.
Wait a minute, you don't have to imagine!
In stock and commodities markets making an exchange in the nanosecond can mean the difference between making a six figure salary and a 9 figure salary and if we want BTC and other cryptocurrencies to be accepted into financial markets we need to avoid building problems like variable transaction fees into our systems.