There is a pretty consistent high CPU usage across Chrome, Firefox and IE11, and all their profilers show it's spending all the time making WebGL draw calls. I suspect it's simply that you are using a very many small tiles. You have seamless mode off which helps, but you still have a huge number of 8x8 tiles with sections of alternating patterns, so there are no runs of the same tile. See How the WebGL renderer works: normally it can render entire regions of tiles with a single "set texture" then a single "draw quad" call, but alternating tiles forces a "set texture" then "draw quad" call per tile. With such small tiles and a large viewport you can fit in 100x100 tiles or 10,000 tiles on-screen, which gives you an idea of how much scope there is to hammer draw calls.
If nothing is moving in the layout C2 does not bother re-drawing the screen at all, which is probably the only reason you see higher CPU usage when things start moving.