What could cause a temperature variance between our kitchen and the rest of the house?
Increasing the fan speed is an indication of a not-very good furnace serviceman.  Whatever that did, it did it everywhere in the house, so the kitchen would have the same relative temperature.

An energy consultant can help you, and bring an IR camera that can "see" if a wall is insulated.  You'll also want to "balance" the vents, which means turning down dampers in overheated rooms.  You can sometimes add floor insulation to bring up a particular room's perceived temperature.

The space under your cabinets might be extra cold, as it may be bare floor boards with no finished floor.