Purely as a user and not developer, there are some bits of the App Store app that suck.
The biggest problems are the Updates and Purchased sections. I admit I do have a bunch of apps in the iTunes account, so there are 500+ apps. But really 500 is nothing--gmail instantly tells me I have 75,174 emails archived.
I timed it: on a new iPhone 5, on my fast WiFi network at home, tapping the "Purchased" item it sits working with the little spinner going for 37 seconds before showing the list. Many times I've thought it had crashed or simply given up. Way, way too long.
The Updates isn't quite as bad, it loads within a few seconds, but still not as quickly sometimes. But if I go and tap "Update" on a few things, it becomes slower and slower to respond. After 5 or so taps, it seems a second or more between my tap and when it shows it recognized it. After a few it gets unusable so I don't ever update more than a couple at a time.
Apple, you can do better.