Low FPS when gaming on AC power

I'm playing Hogwarts Legacy plugged into my Sky Glass via the bundled USB-C to HDMI adapter 

I'm getting horrifically low FPS (20-30) when I have the AC adapter plugged in. As soon as I unplug it, the FPS shoots back up to 60

I suspect it's a CPU issue as the clock speed seems to drop to <2GHz when the charger is plugged in and changing the graphics settings in game doesn't seem to change anything 

I cannot update the graphics drivers via NVidia as that driver causes continuous blue screens when running on battery power, so I have to use the latest driver from Dell's website