I spent about 3-4 hours with a Sapphire RX 580 Pulse yesterday and, honestly, I didn't notice any less lag or more smoothness using it when compared to my GTX 1080. In fact, I saw some lag with the RX 580 when using Mission Control that I never saw with my GTX 1080.
I'll try with a GT 630 or GT 640 later today to see if I can replicate what you guys are talking about.
What are you doing to do these tests?
Please open Photoshop, Illustrator and Premiere Pro and make sure they are all using the GPU. Do some heavy workloads in those applications.
While you have them open, open another WebGL demo in Google Chrome (make sure hardware acceleration is on).
Keep using these apps over and over watch the VRAM fill up in iStat Menu. These are real world scenarios. I don't think anyone runs macOS with just a safari window open and nothing else. Some of us have 10 core systems in the works for macOS High Sierra and have higher end parts.
On fresh boot nVidia runs smooth but over a very very short period of time it starts to lag.
A reboot fixes it for a minute, but when you're doing work every day on macOS, nVidia GPU is useless in High Sierra for now.
I'm on iGPU doing same exact tasks as before and everything is smooth. I can't get my second monitor to work but that's ok for now. I'm moving to a RX580 until Apple fixes this.
I will let people know here that Apple is aware of this, and they are doing some tests at the moment. I hope a fix makes it into .3
FYI I have a clone SSD with Sierra installed and it runs PERFECT on this same exact hardware.