Great Scott. It really does look like 1666 keys per second.
You need to find a way go eliminate all frames between timed intervals.
I would start with 60fps, so eliminate frames every .6 of a second. (Or every 60/1000 (6/100) 16 milliseconds approx).
Not sure exactly what the best way to just remove the excess is, you may need to hit Blender artists for ideas on that if google doesn’t present immidiate answers.
On a 30 sec. dopesheet you should still be able to see the individual keyframes even if zoomed out to have it fit the whole screen (home key shortcut).
also, within blender you can probably get away with 1 frame every .25 seconds if you use curves properly in the animation process.
I know you are dealing with some mocap data, but the theory still applies…