Why do my pre-cooked hams always take way longer than they should?
Apparently the results you're getting are more or less by design. The How to Bake a Ham page on Smithfield's website gives similar instructions to the one you're following:


  Preparation
  
  Genuine Smithfield and Country Hams (Cooked)
  
  If you have selected a fully cooked ham, further preparation is unnecessary. Simply slice
  and serve. If you prefer, reheat in aluminum foil on low heat (275 degrees) until
  slightly warm.


So if you want it significantly hotter than slightly warm you're going to have to do something different than what they suggest. You could try raising the temperature, the USDA recommends a temperature of not less than 325F in order obtain an internal temperature of 140F.