Okay, I got it working and the framerates more than quadrupled!
Here's what I did:
1) update graphics drivers (always do this first, of course)
2) Open Nvidia Control Panel, then in "Manage 3D Settings", there is a dropdown for "Preferred Graphics Processor". I changed that from "Auto-Select" to "High performance NVIDIA processor"
3) From that screen, there's also a link to "Windows Graphics Settings". From here, click "Default Graphics Settings", then turn on "Hardware Accelerated GPU Scheduling"
It seems that both windows and nvidia were assuming that CTA5 was not going to need GPU support, so my laptop was limiting its access to the GPU, which is a good feature when it gets it right, but CTA5 definitely needs the boost.
I don't see a way to change these settings specifically for CTA5, which would be ideal, since now I turned off power saving features that could be beneficial while running other apps. Is this a bug? Is there a way for Reallusion to tell Windows that it requests GPU support on machines with power-saving settings turned on? I'm not versed in that side of programming, so I don't know. But if Affinity Designer got access, what did they do differently?
Anyway, hopefully this helps someone else who is getting poor framerates on a laptop!