Well, it's dependent on material.
AR15 receivers are made of aluminum while suppressors are typically made of stainless steel at the bare minimum. Often, Inconel is used for the core/baffles for overall durability. Comparing aluminum and inconel, aluminum is like butter. Nickel-based alloys like inconel are a PITA to machine.
That said, I do have some suppressors in the works.... :D
2 hours on a lower - that's too high. 45 minutes is about right, perhaps faster if you know how to optimize programs.