Help with performance hit on laptop when gaming on external display

I've recently switched to CachyOS (KDE Plasma) from Windows 11 on my laptop with an Intel CPU and an Nvidia 4070 GPU. I installed the CachyOS gaming packages and have been happily playing RE2 Remake on it at native res +HDR at around 60-70FPS.

Yesterday I decided to plug it into my 4K TV using a USB-C dock and noticed that the FPS was quite a lot worse (about 45 FPS) and extremely choppy. I double checked that the game was set to fullscreen 1440p for the TV (my laptop is 2880x1800). I tried using the HDMI port on the laptop but saw the same results. Disabling HDR had no effect but if I set the desktop resolution to be 1440p the issue goes away. (This is what I'm using right now as I look for a way to have a regular 4k desktop but the game render at 1440p.)

I enabled mangohud to see if the GPU clocks were different but they looked the same. Here, I did notice that mangohud was reporting the resolution as 4k even though the game setting was set to 1440p. I'm guessing this is some quirk of Borderless Fullscreen on dx12?

I tried gamescope as I understand it is something that can help in this situation and run the game at the resolution supplied instead of the native desktop resolution. Unfortunately that didn't work either. I tried with-w 2560 -h 1440 and -W/-H set to both 1440 and 4k numbers.

I tried looking for a solution but haven't been able to, so I'm hoping someone here might be able to direct me to a solution.