I guess that's not really performance, but I agree that UX is one of those things that can subtlety annoy someone enough that they'll just give up and go elsewhere. That's a pretty extreme example, but even simple things like asking the user for too much information over too many pages at checkout can dramatically increase bounce rate. If a user can't land on your site, find what they're looking for then checkout within a few minutes, something is wrong.

On payment failures, an interesting solution an ecommerce I used to work for came up with was just to place the order on payment failure. They figured it was better just to send out an email after the fact asking them to try again and if that failed they could call and take the payment over the phone if need be. We targeted an older demographic and sold fairly pricey products though. I guess that model wouldn't work so well if you're selling $10 tshirts. I believe Amazon does something similar. I know they've sent me emails in the past letting me know my payment failed after placing an order.