I managed to significantly improve the lag on my iMac (late 2013, GeForce GTX 775M) by installing the previous version of the Nvidia web driver (WebDriver-378.10.10.10.25.106). The current version of the Nvidia driver is awful, and is much worse than even the standard OSX driver.
You need to use a tool to force OSX to install the older driver even though it technically isn't meant to run on 10.13.3. This tool (in German, unfortunately) will force the system to install the older driver:
https://www.hackintosh-forum.de/ind...eiber-all-Version-update-App-für-High-Sierra/
Although it is meant for Hackintoshes, this TonyMacx86 thread is the best place for details on this process:
https://www.tonymacx86.com/threads/macos-10-13-3-update.243852/
The graphics performance isn't quite as good as it was on 10.12, but really not that bad overall. It was almost unusable before.