Say it's 25C outside and 30C indoors. I want to bring the indoor temp down to 25C, but opening all the windows barely makes a dent. Does it make sense to have the AC turned on and set to 25C while I have the windows open? Or should I be closing them anyway?
Yeah, the humidity is a very important factor here.
Latent heat of vaporization causes some very non-intuitive temperature physics when you start condensing or vaporizing any fluids. Fun fact: A 300W(electrical) dehumidifier functions as a ~1500W(thermal) heater. And it doesn't necessarily feel like one, because our perception of heat is intrinsically linked to humidity. But that only happens if the air is very humid. It's the reverse of an evaporative/swamp cooler, which likewise only works effectively if the air is NOT humid.