Dell XPS 9560 with i7-7700HQ and 24 GB of RAM. I don't use the NVidia card but the Intel built in one.
YouTube running, 8 CPUs, average of 9%.
YouTube closed, all the CPUs drop to around 3%.
I'm pretty sure it's something related to the NVidia drivers. Maybe in the way they interact with Budgie, but definitely not a problem of Budgie itself.
Edit:
I've around 50, or even more, cards open (or standby) in Vivaldi, IntelliJ IDEA (that eats up to 4GB or RAM), Email and the system monitor.