The internet providers are never going to throttle your specifics, they'll just generally throttle your speeds all around...so if it's only the app store DL's you're having an issue with, it's probably going to be something more local. There are a few things I would try...
If a simple hard restart isn't fixing this issue (holding power and home down until phone restarts), continue below...
- Reset your network settings (under general settings and 'reset'...make sure you have any network passwords you use before hand because this will cause you to have to re-enter them)
- It never hurts to reset your modem and router...and do it in that order...unplug both, let your modem initialize and then plug in your router and allow it to full boot up before trying to connect to it.
- If you have a router capable of multiple networks (2.4Ghz and 5Ghz bands), try switching to the other network (whichever one you're not using)...sometimes the iPhone (in the past) has had issues connecting to the 5Ghz bands, so depending on what firmware you're running, you could still be experiencing this (the most recent firmware should not have this issue)
...try those things and see if you're still having problems. Outside of that, sometimes the app store and iTunes are just slow (especially if they are loading new things to them, updating the versions of them or just generally doing maintenance)...if this kind of poor performance continues after you've done all of these things, and you're not experiencing at any specific time of day (evenings are usually provider "peak" hours and can cause slow downs), you may have to schedule a genius bar appointment at an apple store to see about getting your iPhone replaced.