Updated from 10.13 to 10.14 (so no clean install of 10.14).
I can only say: totally zero issues for me, also totally no lag in keyboard and mouse inputs, also not in Reaper.
My system just flies and feels even snappier than OSX 10.13.
I am totally happy with OSX 10.14.
Maybe install the Combo update 10.14.2 and see of this solves things:
https://support.apple.com/kb/DL1987?locale=en_US
An OSX Combo update (against an OSX "Delta" update available through Apple Store) installs all OSX system files again, so any corruption of current OSX system files on your system will be undone.
Macbook Retina 2015.
OSX 10.14.2