Your setup is well off enough to get decent FPS. Make sure you are on Lowest quality and on Unlimited framerate.

Cube Stacking requires more than just dropping the cube while you are on top of it.

Step 1: Go up against a wall with you back against it with cube in hand: https://imgur.com/a/mxiRg

Step 2: Place the cube where you are inbetween the cube and the wall. The cube cant be too far away nor to close to the wall. I recommend finding a setup with the wireframe of the cube and your FOV. https://imgur.com/a/y1FfD

Step 3: Grab the other cube and go on top of the cube with your back to the wall. http://imgur.com/a/I1ESl

Step 4: Jump and Place the cube at the same time. https://imgur.com/a/4OQzT

Some cube stacks are easier than others and there is also other ways but this works anywhere.