Both of your GPUs have some issues in Zwift right now. If you use the Nvidia GPU, you should employ this workaround:
If you use the Intel iGPU, you should install this driver:
There are some tips for doing the Intel driver install in that topic, if you run into problems.
If you upload a game log file to trainerdx.com, that will confirm which GPU was being used when Zwift was running.