Is the same as in many other threads.
Quite simply - you are using a hybrid profile that by default uses the iGPU (amd), while at the same time you are plugging in your monitor to the dGPU (nvidia).
There are a number of solutions to this … see for example this thread and the responses there
optimus-manager is largely considered trash.
If you need a ‘switcher’ utility then alternatives exist like envycontrol.
But we should probably make sure to deal with whatever mhwd problem you may be having;
So can you please provide the actual logs here?
I can suggest sorting mirrors first as well;
sudo pacman-mirrors -f && sudo pacman -Syu
sudo mhwd -i pci video-hybrid-amd-nvidia-prime
PS.
This guide will also cover things like how to format code etc