Official OpenELEC has never released a version for ODROID-U3, so you probably have used an unofficial OpenELEC portage to ODROID-U3.
I'm using the very same patch for Kodi to support MFC hardware decoding in this unofficial LibreELEC portage to ODROID-U3. Your high CPU usage is not reproductible here.
Yes,it was unofficial OpenElec release. And developer of the that have image is referring to your image now.
Do I understand correctly you don't see same issues in your case?
But strangely I see same issues on regular h264 encoded videos as well, occasional frame drops, high load.
Lurking around odroid forums I have seen developers clames having problem with implementation of Kodi for the platform. And since we don't see 17.1 for the U3 I assume the problem is still there. They have released 17 for C1 though, but that is a different story.
Any chance eMMC image might differ from SD version?