What's the longest wait you had for a transaction to go through? When the spam attacks were running people who paid fees were waiting over half a day, and the backlog took a week to clear. Did you try sending any zero fee transactions while those attacks were ongoing?
Longest wait was when I tried to buy my first Avalon, they wouldn't give me an order number until my transaction confirmed, I was watching people chime in with order numbers going up and up, felt horrible, I don't remember exactly how long it took, I waited a couple hours, then went to sleep, so sometime between 2 and 6 hours.
I did not try any 0 fee during the height of the stress testing, in hindsight I wish I had. I did send a couple with minimum fee that were confirmed in first or second block.
Even without a fee I suspect they would have been confirmed pretty quick just because of the age of the coins and because they are from mining so typically very close to their respective genesis blocks and they are typically from a single unspent output...
Doesn't say much for fungibility, but not all coins are considered equal unfortunately. Also not great for scalability as the more the coins become split up, the more they are exchanged back and forth, the more difficult they become to move...
Edit: in fact, if this bitcoin thing continues to succeed for the next several years at some point I could envision that old coins in big chunks will sell for a premium to payment processors simply because using these less tarnished coins could bring considerable savings in transaction costs...