I had a 3 Million Vertex model in Blender loaded, and handling it in Blenders viewport was agonizing even in Object mode. Until I went to the settings and enabled VBOs in the open GL section ... after that, lightning fast again. Probably everbody but me knew that
I'm not sure if the inability to select the GPUs as the calcualtion devices in our newest Octane Blender versions has anything to do with Viewport performance? I guess not and that is only used for Cycles etc?
Sources:
http://blenderartists.org/forum/showthr ... erformance