you can put a 40000 faces mesh, but if you put a lot of 40000 faces meshes, old computers will reduce the FPS (frames per Second), or bad saying "LAG"
For example, I have a mesh with 80 triangles, i use it 100 times (80x100=8000) doesn't reduce to much the FPS.
Also it depends the screen resolution, lower resolution will increase FPS (reduce video lag).
I have an old computer (7 years old), with a graphics acelerator nvidia GeForce 5500 Fx (old graphics card), bonfire make me to suffer with 1 to 4 FPS even at lower resolution.
You need to test yourself how many faces is a lot, having consideration on medium old computers. "if runs fine on your computer, it will run acceptable in mine".