I have a superflat world with no particles on it. None at all. I press F3, the particle count is absolute 0. Then when I create a repeating command block with a small particle command in it that creates 100 particles per tick, the particle count rises to about 1600 particles in the F3 menu. If I block off those particles using solid blocks, the particle count goes unchanged, even though those particles aren't actually visible. Same for when I just completely turn away from the particles, and they aren't even inside my field of view.
It might reduce lag if they would get visually unrendered when they are not visible to the user.