Blender Performance in Preview mode

I would like to ask about some overall Blender performance issue when Render Preview mode is enabled.
In my system I use Internal Intel HD video adapter for display ( and OpenGL ) and two GTX’s ( TITAN 7xx and 1080 ) for rendering only. But when preview is enabled my CPU works on 40% and whole Blender interface has lags.
I use also Thea Render and Octane and have no problems with Blender UI performance when preview rendering is enabled.
I use PBR shaders.

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