Hi,
First, adjust the temp controller to min. you're currently driving with the heater slightly on. ! the frequent turning on of the compressor may be due to previous owner reducing the cooling to save fuel. ever considered a light window tint?
Yes the a/c system puts a noticeable strain on the engine,,. switch off when you need extra power. and wash the condenser when your washing the car to maintain cooling efficiency. make sure the cooling fan freely runs with the a/c on. |behind the rad
And keep the interior re-circulation fully on,..
p.s. add a pic of the a/c control panel/,.