Maybe the process is more cpu then gpu intensive, all depends what is done on the timeline.
On a side note you should turn off Frame Blending that is only usefull when changing framerate.
If you have MPE hardware on no need to check Render at Max.
Your export is also going from progressive to upper, is that correct?
As a general rule of thumb, AMD CPU's are not worth considering at all, since they suffer severely from the very limited and badly implemented support of SSE 4.x instructions, which are widely used in PR. That makes these processors very slow, even in the 8-core versions. You get what you pay for, and with AMD that holds very much. Even though Adobe claims AMD processors can be used, that is intended more to make PR look attractive, but it really means you can install the program on an AMD machine, however you can't edit effectively with such a CPU. Even the latest octo-core AMD's are significantly slower than middle-of-the-road Intel quad cores. If you have an AMD processor, do not expect to edit comfortably any codec more demanding than DV. All phone or action camera formats are way beyond what an AMD can handle.
Add to that the R9 280 video card, which is about 2 times slower than an equally priced nVidia card, and it does not come as a surprise.
How many processes do you have running?