I'd like to link my findings here, which I've posted as a ticket on NvidiaGraphicsFixup hoping to enlist lvs1974's help:
https://sourceforge.net/p/nvidiagraphicsfixup/tickets/10/
I've also found reports of the lag occurring on real macs with Nvidia cards over eGPU:
https://egpu.io/forums/mac-setup/wip-nvidia-egpu-support-for-high-sierra/paged/25/
My best current theory is that the lag is most severe on Non-Nvidia SMBIOSes.
Per Nvidia's site, the Nvidia SMBIOSes are: MacPro5,1, MacPro4,1, MacPro3,1, iMac14,2, iMac14,3, iMac13,1, iMac13,2, MacBookPro11,3, MacBookPro10,1, and MacBookPro9,1. If you are using any of these, your lag is non-existent or minimal. It doesn't seem to matter what type or family of Nvidia card you use nor does your SIP status seem to matter ultimately. No tools, kexts, or other fixups seem currently able to resolve the lag.
It is possible that Skylake+ systems have worse lag. The lag is proportional to resolution, and the high CPU usage by WindowServer suggests some sort of falling back on software rendering, but it is not a library validation issue. Transparency all exists where it should. The issue is lag, occasional artifacts, and display detection failures, especially after sleep and/or display idle off.
I hope this info is helpful and that Nvidia will release an update and/or someone is able to find the source of the issue.
Thanks!