Each OS will prevent this. Because there are more processes/threads running then cores are available the ffmpeg thread and even the whole XBMC process are very likly to get moved off the CPU by the task scheduler.
AppleTV4/iPhone/iPod/iPad: HowTo find debug logs and everything else which the devs like so much: click here
HowTo setup NFS for Kodi: NFS(wiki)
HowTo configure avahi (zeroconf): Avahi_Zeroconf(wiki)
READ THE IOS FAQ!: iOS FAQ(wiki)