Probably because all video decoding & acceleration has to be done by the CPU, and not the video graphics card.
Gallium 0.4 is some form of ATI/AMD graphics if I'm correct. Support for those have always been troublesome. I would stick to Mint + Kodi.
Actually the load was always high even when idle, and video playback adds almost nothing to it.
Found out this is actually Intel GMA500, Gallium is some unified Mesa 3D interface.
says that any system with gma500_gfx module included should work. And that hardware decoding is supported.