Have you checked if these devices are actually capable of playing these files, period? And if so, have you looked into Kodi's hardware acceleration?
To have smooth playback, you either need enough processing power to just do it, or hardware chips optimized for playback, in addition to software support to use those features. If the hardware can handle it but you haven't told the software to try, it's not surprising you're having issues.
I assume the devices are capable because the Jellyfin app plays the file without an issue.
And the jellyfin server shows Direct playing whether i use the jellyfin app or Kodi.
I'm not aware what hardware acceleration in Kodi is, but will take a look.
Quote: If the hardware can handle it but you haven't told the software to try, it's not surprising you're having issues.
I would buy a NVidia Shield, but can somebody help me verify that the issue is really in the client device and not in the server/network before i buy it?