Reason for dropping framerates

I am currently playing on a Laptop with the following specs:

i7-8750H, GTX 1050Ti 4GB max Q, 8 GB Ram, running windows 10.

So I wanted to investigate if I would benefit much from a better graphics card (egpu). So I monitored my FPS and hardware usage using MSI Afterburner during the quickstart FA18 CASE I Landing on the Caucasus map, in windowed mode. I then plotted the data in a scatterplot, CPU_max means, it shows the most used core at the time.

Using the 1440p resolution, I could see and feel that I can not reach the framerates I want in such a low demanding setting, so I did not look into that any further.

1440:

But now to the 1920x1080p results. I can see that the graphics card seems to be the limiting factor, as the other components don't seem to be maxed out. But you can see that, when the FPS drop, the graphics card usage drops as well at times, without one of the other two components seeming to be the limiting factor. For example, when the FPS drop to 44, the GPU usage drops under 90% as well.

1080:

Do you have an idea why that is? Thank you!