The person you're replying to could have meant that different foods have different glycemic indexes [1]. Essentially, while foods may contain the same calories as measured by a bomb calorimeter or similar, the rate at which those calories are broken down by your body into simple sugars for absorption by your blood varies. A "slow" calorie in that sense would be something with a low glycemic index, like nuts, which would cause a much more gentle rise in blood sugar.
There's evidence that a low GI diet can reduce the incidence of type 2 diabetes and heart disease.
There's evidence that a low GI diet can reduce the incidence of type 2 diabetes and heart disease.
[1]: http://en.wikipedia.org/wiki/Glycemic_index