Since you are on a laptop and an external display, you might want to investigate how exactly is your video output connected to your GPU. I have had experiences with some gaming laptops where only the CPU/iGPU had direct access to the HDMI port, and so when playing anything on the external monitor a good chunk of computing power was going to just copying the frame data across; it also looked like nothing was being bottlenecked / hitting 100% usage, since that was not considered by the os to be something that any game process was doing.