Why does my iPhone receive messages late when I'm in a group text?

I have the iPhone 4s with 16g (I have less than a gig left of space) and am on T-Mobile.
Every single time I'm added to a new group text, my phone delays all messages sent to me. Regardless of whether it's another iPhone or if they're on the same carrier as me, none of my messages get to me until an hour or so later.

Is it because I'm low on space? Or is it T-Mobile (probably the worst carrier I've ever had by the way)? Or is it the iPhone software?

Probably how Tmobile is routing texts to you. Remember a tower in your area is handling hundreds of transactions at any given time. Some transactions are put in queue like cars in traffic on the freeway. Especially if the group text is having to dole out to several recipients at once. Also iMessages run through Apple's servers and not Tmobile's towers. So if you have issues with that too, then I'd try this step below. Hope you see improvement. Best luck.

http://osxdaily.com/2010/01/11/fix-iphone-cellular-data-problems-by-resetting-network-settings/

http://support.apple.com/.../kb/ts3780

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