You mentioned that your cpu is getting maxed out on wireguard. That makes a lot of sense since it’s generally not hardware accelerated, old low end CPUs could struggle here.
What choices do you have for protocols with your VPN software?
A quick skim of this tells me it’s a bit more complicated than one or the other. nto.github.io/AirPlay
It would appear to me that if you’re doing a traditional casting of the video it’ll not use your VPN. As it just passes through the information needed for the device to fetch the video. Unless everything is configured to use the VPN*
Haha, if anyone reading this ever finds themselves in dire need of the contents in a can without a reasonable tool. You can grind down the top of the can on concrete until the top pops open.