Have you checked with AT&T either through corporate customer service or with a local AT&T store. It could be what AT&T calls a degraded tower. An indication of a tower problem is if your phone switches between H+ and E which will also drop calls if you are on the phone when this occurs.
Do other phones have the same slowdown? There are still a large number of complaints and concerns that the iphoney is given access to greater speeds than real smartphones so that might not be a good comparison.
Just in case did you accidentally force 2G? Settings > Wireless & Network Settings > Mobile Network Settings > Select Network. Select network should be greyed out. Might also want to check which APN is selected.
When you set up your atrix did you use a new SIM card or use your prior one? I was given the option to use my older 3G SIM with the explanation that the AT&T party line was to require using a 4G SIM but that the reality was that the 3G would continue to work for an unknown period of time but eventually would require a 4G SIM. If using a 3G then it would be a good time to switch to a 4G.