These are 128 raycasted 3D metaballs with quite some optimization even though you could propably squeze out another 50% performance increase.
It uses spatial hashing on the GPU, bounding spheres, aswell as adaptive size stepping and empty space skipping to make it run in realtime.
I get somewhat from 5-10 FPS on my 4 year old Mac Book Pro, so on modern GPUs that would be something inbetween 30-60 FPS.
Just in comparison, this runs faster than the 6 metaballs from my first video.