I use progressive mode all the time with a GPU but some people have pointed out this is a slow method to render, below is a suggestion why

I'm assumimg here that the GPU should run at 100% all the time to get the best render times.

Metrics from the tests run on a 6700K + GTX1070 - underclocked to get 800MHz. Graphs from MSI-Afterburner

Using "Render to Background" appears to run the best with minimal drop (not tested but "Render to Queue" should also work well)
Standard Render (non progressive) has a small performance hit which increases with slower CPUs.
Progressive mode has a significant performance penalty particularly when rendering large images

CPU speed - GPU Utilisations 4.3GHz vs 0.8GHz

Progressive Mode and image size. This effect is not significant with the other two render methods