@mhagain: Not large at all, just a small smoke from bullet impact, when I draw some smoke on different bullet hits infront of the screen, I see well noticed slow rendering.
This means that whenever the player is shooting, the rendering will slow down due to smoke particles.
What frame rate are you getting in each case? Drawing extra stuff is never going to be free.
How many particles get rendered? With how many draw calls? How many pixels does each particle cover on average?
How long is the CPU taking to update all those particles and copy them into the vertex buffer?
Is it any faster if you use a managed vertex buffer instead of a dynamic one? (normally this is a bad plan, but in some cases it is quicker)