Rendering performance with increasing number of lights

I have a unity scene with a big city model. I have created an unlit shader that applies forward pass rendering and calculates diffuse, ambient, and specular components. Please see the attached screenshot of the scene. I tried to plot the results with an increasing number of lights (5, 10, 15 and 20) and was hoping to get a growing linear trend in GPU duration per frame ( as I thought with an increasing number of lights, the number of passes increase and thus the GPU workload increases). However, it doesn’t appear valid from my plot. Contrarily, I get a downward slope. I have plotted the number of lights on the x-axis, and mean of GPU duration per frame on the y-axis. Could anyone please suggest what I might be missing here? Thanks!

Note: I have performed these runs on my laptop with Nvidia 1050Ti GPU. I measured the GPU duration per frame using Nvidia Nsight visual studio tool.!
NVIDIAGeforceGTX1050Ti_UnityFrames_lights_Q|690x458 !