Collider Cost to FPS question

Q1: i want to know if using mesh colliders cost more in terms of FPS and overall performance?

(example: i have a tree in a spherical shape should i use a sphere collider or a mesh collider on the spherically shaped mesh)

N: i am guessing mesh colliders cost a lot more but i'd like to be sure first.

Q2: also since we're on the subject can i use a terrain collider on my land mass i created in blender or should i just use a mesh collider?

Q3: i am making a survival/capture the flag low poly game so if anyone has any other tips to optimize my games performance it would probably be overwhelmingly useful to me.

Thanks in advance.