How to check if my iPhone's connection is being throttled?
Whether your internet connection is slow may have several reasons:


poor, instable connection
geographic location
total traffic on the network
you've reached your cap (see contract)


For the normal users it's difficult to determine what the exact reason is. Especially in dense areas, the promised speed is not reached sometimes because the total traffic of all users is very high. Where I live, the same network is used by different providers, adding additional complications.

In order to judge if your network connection slower than usual, you should test and record your network speed for several times throughout a month.

You can use Mobile Speed Test to get stats about Download, Upload and Ping. You can use this data to check the speed you've been promised by your provider. Check your contract to see what speed you have been promised. Most contracts throttle the speed after you have reached a cap.