Why does my iPhone work so slow on T-Mobile?

I got a iPhone and was on AT&T and now i'm on T-Mobile, I want to keep my iPhone but it doesn't work nearly as fast as it did with AT&T. Is there anything that i can do to make it work faster?

Thats because its t-mobile and they really suck. I would change back to at&t or go with sprint. I had t-mobile before and wow its horrible and staticly and had so many problems.

It is probably so slow on T-Mobile because T-Mobile is not an apple authorized carrier, whereas AT&T is and apple authorized carrier

Slow meaning apps run sow or slow meaning data is slow?

Data may be slower because t-mobile use different frequency bands than at&t for their data network. If you buy the phone from AT&T it will come with a radio that will work with the AT&T bands but they won't worry about the other ones.
The iPhone will use the fastest network that both it and the phone company support, depending on the model of iPhone you have and where you are in the country that may mean you're stuck on a 2G Edge network.

If you mean that apps are running slowly or data is slow when you are on Wi-Fi then I have no idea, the phone company shouldn't have any impact on that at all.

The first two answers were totally unhelpful, in addition to being wrong.

The reason you phone is "slower" is because AT&T and T-Mobile do not use the same carrier frequencies for 3G and 4G data signals. Call and texting work OK, because they use common GSM frequencies.

But their high-speed data networks do not.So, you will not get anything about 2G/EDGE-class data signals.

If you had asked someone before you made the switch, you would have known this because it's common knowledge.

This is the problem with GSM carriers in the US. In Europe and other countries, most of the carriers share the same frequencies, so it's easier to interchange phones.

If you want fast, go back to AT&T, or sell the iPhone and get a decent 4G device that will work with T-Mobile's 3G/4G network.

T-Mobile's 3G bands are not supported on iPhone so you're getting slower internet on EDGE (2G). You can use Wi-Fi for faster internet when you're in Wi-Fi hotspot area. T-Mobile will be getting the 1900MHz for 3G-4G services so wait till it's released, you'll get 3G on your iPhone.