not sure you have a problem or not. If your home was sitting idle in 91 degree heat before cranking up the AC it's going to take a long time to drop the temp to 77 especially with a single 13.5 BTU unit. Everything in the unit will need to cool down. Walls, furniture, appliances all retain heat. If you can test the air temp at a return duct and at a vent. If the air going into the return is 90 and the air coming out is near 70 then then AC unit is doing a decent job. But, pumping 70 degree air into a 90 degree house will take a long time to cool especially in the direct sun. If the AC unit keeps things nice after the initial cool down then don't worry about it.
