[Kodi 17] This is weird: External Player CPU Usage

Setup: i5 NUC running Windows10 64bit and Kodi 17.0 with MPC-HC 64bit as external player for all media.

This has me baffled:
When I play HEVC .mkv encoded content launched via Kodi, MPC is launched as normal but playback is super-choppy with audio dropout. CPU usage is 100%. There are no command-line parameters or anything unusual.
If I launch the same video directly in MPC it plays perfectly fine with a 35-45% CPU usage.

Why would there be this massive performance difference using exactly the same player and how do I fix it?

Huh