Linux 4.14 introduced a serious performance regression in the i915 driver where eDP displays supporting 60hz would be limited to 40hz

The regression was caused with this commit
https://github.com/torvalds/linux/commit/dc911f5bd8aacfcf8aabd5c26c88e04c837a938e

Upstream bug report
https://bugs.freedesktop.org/show_bug.cgi?id=103497

Discussion of the problem affecting various distros
https://forum.manjaro.org/t/poor-opengl-performance-on-linux-4-14/35453/155
https://github.com/NixOS/nixpkgs/issues/31999
https://bugs.archlinux.org/task/56711

Also affecting Solus users
https://plus.google.com/108338039810238937706/posts/5mo61Smp3rR

The regression can be fixed with this commit
https://bugs.freedesktop.org/attachment.cgi?id=135277

It has not been backported as of yet so please apply the patch or revert the original commit for the next kernel update, cheers!