NOTE: I am NOT a USAA employee. The facts listed below are as of when I worked at a bank and may have changed over the years. However, I believe the basics are still true.
I wanted to point out a couple things. I worked for a major bank for about 10 years and there are some "behind the scenes" things you are not taking into account.
1st: With @home and @mobile, USAA does not have the instrument in hand. You COULD deposit it at several banks at the same time. Therefore, the hold until it actually passes the paying institution. A GOOD check, as you said, passes quickly. But all checks are not good. If you could only imagine the amount of fraud the banks have to deal with.
2nd: If the check is not good, USAA would get notified within a day or two that the funds were not honored. However, the actual instrument has to be returned to USAA from whatever bank your funds were drawn off of. This part is not electronic so it takes a few days. Hold times are a "standard" for what an "average" processing time takes plus a day (normally). Your payment with a good check may not need that much time but USAA has no way of knowing if all your checks are good until they are processed.
3rd: Keep in mind that banks need to make money too. Yes, they probably make a small amount on held funds but on $100 or $300, how much do you really think they make? Would that be worth customer complaints without some reason behind it? Unless or until you actually work in a bank (assuming you have not) and work in an area where processing and decisions take place, you might want to consider there are things going on that you do not have available to you.
I hope that helps explain some of the hold times.
