Does using a monitor with a laptop decrease FPS

I recently bought a Surface Book 2 13.5 for uni and the occasional gaming. I really only play Overwatch and I have been suffering major FPS issues. I connect my laptop to a external monitor AOC U2879VF via a USB-C to HDMI adapter. It has a 1050 GTX and i7-8650U CPU. I have a fan blowing onto the back of the laptop [on the Windows logo] and charger plugged in with the slider at Best Performance when I play.

When I play on the laptop monitor, on 1920x1080 75% render and all settings on low/off with 300 FPS cap, I easily get above 100 FPS. GPU usage hits 99% and CPU hits around ~65% with both hovering around mid 50°C.

However, when I connect to my external monitor my FPS decreases dramatically with the same settings. The GPU hits 99% but then decreases to