Mesh causing slowdown?

I got this file in FBX. I did it on Maya.

Now when I import it to Unity and I change one of its settings on Inspector, like "Generate Colliders", it takes some time to apply.

And when I actually play to test it and goes walking through it, in some parts it freezes for an instant and then goes back to normal, like skipping frames or slowdown.

I don't know exactly what causes it and how to get it fixed. Maybe the file is jsut too big (18MB) and I have to delete some polygons?