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?
You provided an image of a mini split. They are so efficient, if it is colder outside than in it is pretty negligible that it will be any less efficient. Unless the window is near the outside unit and the heat just comes back in. But you need to take into account the humidity, if it's much more humid outside you definitely should keep the windows shut.