Why does the flashlight not lag, but DynamicLight does?

Flashlight does nothing to my FPS, but DynamicLights bring it down from 120-140 to 60-70.

Isn’t dynamic light more precise than the dumb hl2 flashlight?

The flashlight is simply a projected texture, which is cheaper to render.
A dynamic light is calculated in-game, which means they have a higher processing cost.
Both entities can light up brushes at real-time.
Read more here and here.

And this is why I love you rejax.

I think one of those 'here’s were supposed to point here

Flashlight is fully clientside, and others players just sees a shitty texture.
If you use lamps is singleplayer, you will not lag.

You’re saying DynamicLight ISN’T clientside?

I’m not sure, but i thinks there is a little server side code.

projected textures are dicks though because they corrupt textures for mac (and presumably linux) computers. I’d highly suggest using dynamic lights if you’re not trying to get the lamp-effect.
Also, for some reason the built-in flashlight from HL2 does not corrupt textures, which is kind of odd.

I’m trying to create night vision, but dynamic lights are ridiculously laggy.

It’s very expensive to brighten lightmaps (read: brushes) with dynamic lights, since it’s being computed on cpu or something, but it’s relatively cheap to use them on models. You can disallow DynamicLight to light world tho.