I think I see spider mites on those leaves also. You can see a tiny web between the stem and the top leaves in your photo. This is a common problem with tropical plants in an indoor situation, especially one like yours with heat and low humidity.
Neem oil, which might be easier for you to find, would smother the critters. Be sure to follow the directions on the bottle and re-spray at the earliest interval indicated so you smother all the subsequent generations as the eggs hatch.
Grocery store lemons are likly hybrids and the seeds could follow either side of the family. You never know what you are going to get, it may be that you have a slower growing plant. I am not sure why you pruned the side branches, removing leaves removes a source of energy for the plant. Usually you prune only for shaping.
And, like el Pollo says, water that little tree on a regular basis.
