iOS updates getting listed slowly
I don't think that this is an issue with your network or your device. I have the same behavior on all my 5 iOS devices and so does everybody else I talked about this topic.

I guess the slowness comes from within Apple's datacenter(s). Checking for updates is a "bigger" process I guess (checking your apps, asking the database, etc.) than "just" downloading, which is pretty simple (establishing a connection, download the data). I just guess that checking for updates just needs some more web service requests than downloading. And since this is done by millions of users, the queue may take a while until your request is processed.

So my answer is: I think you can do nothing about it.