Well, now... Given that ON THE WHOLE, the performance of the iPhone 3G after "upgrading" to iOS4 is woefully dragging along at a snail's pace, I would have to say that iOS4, IN GENERAL should not have been deemed as sufficient to "reach the company's standards for performance" -- it CERTAINLY does not meet MY standards.
I'm sorry to hear this: for me, the performance of iPhone 3G under iOS4 does not appear to have suffered significantly, and it sounds like a number of people have had the same experience as me.
It's true that for the first ten minutes or so after I upgraded, things did seem to go rather slowly. But since then, I would say common tasks (flipping through home screen pages, typing, etc.) are at least as fast, if not faster, than they were under iOS 3. Graphic transitions (when closing programs, for example) seem smoother than before. The only thing that does seem a tad slow so far for me is loading pages on iBooks, particularly ones that have images on them. But in my informal testing, I was going through pages a lot more quickly than I would in real life if I was actually reading them.
Unless there is something (caching, etc.) that would eventually slow down the phone for some reason, I think iOS 4 would be very usable on iPhone 3G. I am getting iPhone 4 on Thursday, but will be recommending to my wife that she upgrade to iOS 3. She has much fewer apps and does a lot less on the phone than I do, so it would go to reason that she should also not have a noticeable performance hit.
Specifically about the background images: I think the look of the home screens is actually better than under iOS 3 (with the change to the dock in particular) even without the background images, so I don't think it's a problem not being able to use custom backgrounds -- especially given Steve Jobs' assertion (along with those of a number of people who tried this feature with jailbroken phones) that it would slow things down. I would definitely be frustrated if the OS were sluggish, so I am glad they didn't add this if that would have been the price.
I would be the first to complain if iOS 4 were slow on my 3G, but fortunately it's not for me at least.