I have noticed that when I transition between Wifi, 3G and 4G (and occasionally 1x) networks. I've noticed that during these transitions, my phone becomes very, very sluggish to the point where I'm unable to use it. It seems like it is checking for updates, mail and other information, as if each transition triggers a new check. Sometimes if I wait long enough, the sluggishness goes away on its own, and sometimes the phone needs a power cycle. This is especially annoying when transitioning quickly between different types of networks, such as when I'm riding on the train. Rapid transitions will sometimes completely lock up the phone.
I do have my phone set to not update unless I'm connected to Wifi, but that leads to a separate but related issue. If I'm at a restaurant or other location that has wifi. As soon as I connect, I experience the same sluggishness and the phone becomes unusable for 5-10 minutes due to checking for updates and then auto-installing whatever app updates need to be installed. At home this wouldn't be a big deal, but if I'm waiting for friends to contact me, for instance, and my phone is unusable, doesn't that defeat the purpose of having a mobile phone?
What I would love to see is the user given full control over when checks are performed and when app updates are actually installed. If I could set my phone to only update between the hours of 3-5am every morning, that would be awesome. And if it would not consider a change in network as a trigger to perform checks, that would be great too.