Here's another way of looking at it, using more useful units (total pixels instead of pixels squared, and milliseconds-per-frame instead of frames-per-second):
At 160000 pixels render time is 0.625ms.
At 480000 pixels (3x more) render time is 1.3ms (2x slower).
At 786432 pixels (5x more) render time is 2.5ms (4x slower).

The render time goes up fairly linearly with the number of pixels drawn -- this indicates that the main bottleneck in your application may be pixel shading operations.

Here's another way of looking at it, using more useful units (total pixels instead of pixels squared, and milliseconds-per-frame instead of frames-per-second):
At 160000 pixels render time is 0.625ms.
At 480000 pixels (3x more) render time is 1.3ms (2x slower).
At 786432 pixels (5x more) render time is 2.5ms (4x slower).

The render time goes up fairly linearly with the number of pixels drawn -- this indicates that the main bottleneck in your application may be pixel shading operations.

Or bandwidth, which would also usually scale linearly with the number of pixels being drawn...