IRL mortars reload EXTREMELY quickly. If I'm remembering correctly, I'm not military trained and haven't actually gotten to use one, the reload is seriously just dropping the shell in. The mortar itself would be positioned and set for where you want to hit already. so in real life, the setup would be the long bit, and reloading and firing would be really quick. But as far as I'm aware, we don't have that capacity.
Same would go for heavy machine guns, they take a while to set up and as long as you don't have to move them, basically lock down that area completely.
But I don't think the game mechanics would allow for either of those.
Edit
After doing a little bit more research, the difficulty of loading firing and moving mortars would depend on the style and tech you're working with, modern infantry mortars tend not to have a reload, you drop the shell in, and the firing pin on the bottom sets it off when the shell reaches it. older mortars were far heavier and closer to cannon in style, though designed to take advantage of the extreme arc.
My suggested solution would be to have multiple styles of mortars, the light ones would do less damage but be quick to fire and either have a virtual magazine of shots or have an instant reload, while heavier ones would do high damage but be heavier and slow to reload.