Weird, it works perfectly fine here with the xorg.conf file above and LE 8.0.
I recorded two 240fps videos last week to illustrate the difference and posted them here:
Intel Apollo Lake
As a response to my feedback, the Kodi developer fritsch created a custom build of his HEVC 10bit LibreELEC build with DRI3 enabled and the latest version of Mesa. On my NUCs, this works just as good as stock LE 8.0 + custom xorg.conf file.
You can try this build, as it has DRI3 enabled by default and hence doesn't need any custom xorg.conf file:
Index of /openelec/hevc10/dri3
Just remove your custom xorg.conf file, download the file LibreELEC-Generic.x86_64-8.0.0-dri3.tar into /storage/.update and reboot your system.
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Btw, have you configured your GUI to run in 60Hz? If you choose something lower, you will most likely experience that the menus feel sluggish, but that has nothing to do with DRI3