After discovering this same issue I spent about 3 hours on the phone with AT&T, who then referred me to Apple. Since I really wanted this fixed, I capitulated and when through the restore process to make Apple happy. They agreed and opened a repair ticket. HOWEVER, I started to do additional testing and asked friends to do the same test. (Ensure LTE is off, since it will not be used during the on call data session, use the speedtest.net app and get a base line, then start a phone call, while the call is going run the test and records the results) They recieved the same types of deltas between on call and off call data. Also note, they are on different hardware and OS's, so this is not an Apple problem. Your results will vary depending on area, but one common factor is the masive decrease while on a call. For example,
off call, 94ms ping, 6.29Mbps down, 0.68Mbps up
on call, 234ms ping, 0.54 Mbps down, 0.01 up
That upload speed while on a call is very consistent at 0.01 at all the test sites (about a 40 mile radius in Orange County and Inland Empire [Southern California]). There was one test that came back 0.00, it may not have been able to complete the upload in a timly manner.
I went back to AT&T spent another 2 hours, many transfers later, a ticket was opened for field techs to investigate. Two days later (today) I recived a call back stating that the issue was resloved and it related to visual voicemail, and to power cycle my phone and all would be well. It was not. No change in data speed was detected.
I called back and spend another hour on the phone and was able to get the ticket reopened.
I recommend that you call AT&T and push this issue. They need to know that more people are having this issue. If you do use "MarkThisSpot" you may want to ensure that you have a phone call going when you start the test, as it will do a 30 second diagnostic before it completes the submit. (I hope that data includes a bandwidth test)