ffmpeg blend video frames – seems to be doing excess work?

I’m blending every 16 frames down to 1 to drastically shorten a timelapse video.
However, it runs very slowly. Is it blending every frame with the 15 following frames, then dropping 15/16 of the frames? That would mean it is doing 16x as much work as it needs to.

If so, is there a better (faster) way to blend frames that doesn’t do redundant work?

-vf "tmix=frames=16:weights='1',select='not(mod(n,16))',setpts=0.0625*PTS"