Decrease resolution when using Micro HDMI display?

I attempted to use my GameShell with a monitor for an at-home experience with a bigger screen, but the resolution is too high and it kills the framerate, even on NES games. Is there a way I can use 320x240 resolution even with an external display?

If you’re using Retroarch, you should be able to go into the settings menu>video>full screen mode to specify the size you want if going full screen.
Scaling can also have an affect on this.
If you decide to work in a windowed mode, depending on which OS you’re using, you may have a different window manager, DWM or TWM (from memory)
Just fiddle and resize, preferably with a mouse or similar.
This is probably the route you’d need to take for other standalone emulators, if they don’t have their own internal configuration options. Otherwise, you’ll need fo find the respective config files, and manually edit whatever settings you can.