Performance drop when particle systems occupy the most part of the screen.

I'm using particle systems to create stars in my space sim project. The star is made off 2 particle systems: star surface (max particles: 500) and corona (max particles: 250), both use the Mobile/Particles/Additive shader, and a sphere mesh with Standard material. And it works fine and looks pretty nice, but there's one problem:
When I get close to the star so it occupies the most part of the screen, fps drops significantly (from 60 to 30-40). This happens in editor and mobile (Samsung A8 2018), didn't test PC standalone build yet but I'm pretty sure the result will be the same.
Disabling the sphere mesh doesn't affect the performance. Disabling of one of the particle systems does improve performance. When the camera is relatively far away from the particles, performance is fine. Changing the scale doesn't seem to matter, whether it's a big star with real size radius 10 million units or a small star with radius 1 unit.
The problem occurs when the particles occupy the most part of the screen space. Changing the shaders to other available ones does change the result FPS but whatever shader I use the performance drop is still there.

So, the question is: are there some tips, workarounds or something? Or do I have to write a custom shader? Or changing shaders won't help anyway?
I'm using built-in render pipeline, by the way. Tried URP and while fps is a bit higher in general, there's still a big drop (to 40-45 fps).