I think it depends on how you structured your code, are you triggering the mesh generation at func _ready? Then it's normal.
If not, I'd say paste here some of your code, so that I can help more.
I can tell you that in my case I used properties (export, with onready), and used them to trigger the mesh generation function only when value is changed (using setget), so, for example:
export(bool) onready var trigger_generation= false setget GenerateMesh
if(_newval != trigger_generation):