Diagnosis
That built-up dust on the aluminum evaporator coil acts as insulation, making the unit work way harder and less efficiently. It needs a physical cleaning.
How to Fix It
Unplug the unit. Remove the front plastic grille—it usually just snaps off or is held by a few screws. You'll see the large, finned coil. Use a soft brush attachment on a vacuum to GENTLY remove loose dust. For the packed-in stuff, get a can of "foaming coil cleaner" from a hardware store. Spray it on, let it foam and sit for 10 minutes as per the can's instructions, then rinse it off with a gentle stream of water from a spray bottle or hose on VERY LOW pressure. You must protect the electrical components at the bottom—cover them with a plastic bag. Let the unit dry completely for a few hours before plugging back in.
If That Didn't Work
If the coil is clean but performance is still bad, the other side of the system (the condenser coil) might be dirty too. That requires removing more cabinet panels and is a more involved job.
Heads Up
Do NOT use a pressure washer or stiff wire brush. You'll bend the delicate fins and cause airflow problems that are worse than the dirt. Bent fins can be straightened with a cheap "fin comb" tool.