drawing a mesh pattern is of course faster even than normal drawing. I am a game dev, and what you say is wrong on so many levels.
Here 2 reasons why the mesh drawing of the spot light is faster than normal drawing and 1 reason why it is much faster than transparency:
1 only every second pixel gets filled, makes it already twice as fast
2 the "mesh" has only 1 color, not changing the color is faster than drawing anything else
3 transparency isn't an overlay, to draw transparency the code has to get the color of each individual pixel of the shape, then calculate the interpolation of the target pixel with the overlay pixel and then redraw the pixel. This is always slow, no matter what engine or system, like trash like html5