I believe/hope that I've finally solved it, at least for my Windows 10 VM. After months of ever growing frustration, various reinstalls of W10, downgrades to Parallels 13 and a tremendous loss of productivity I today unchecked the 'expanding disk' setting (see configuration --> hardware --> hard disk --> advanced settings --> properties). And since having done so Parallels 14 runs smoothly again, no lags, I can hardly believe it. Other optimisations/settings that I (still) have implemented, but that individually did not solve the lags were: disabling superfetch and windows search, mirror displays (OSX setting as I use an external display). Unchecking the 'expanding disk setting' will cost some disk space (about 35GB in my case), but that's just a minor inconvenience when compared to having a non-functioning lagging system. Hope this works for others as well.
Last edited: Sep 25, 2018