Screenshots takes huge amount of time to take and are of size roughly 2MB because of huge BG used and the way they're handled. Obvious solution would be to either change screenshot mechanics to save rendered frame instead of re-rendering it to file or decrease size and resolution of BGs, there's no really need to keep almost 4K BGs in 1080p game, and please - compress them? They're saved as JPGs but they're bigger than good PNGs, the hell? Good thing they're not BMP.
Not really a definitive answer (as they will still be the same type of file sizes) but I tend to use the 's' shortcut for taking screenshots in Renpy as it usually has a better response time / also allows me to capture the dialogue box if I want it.
Written this down to be followed up on though, will get back to you when I have something new.
Be warned, elements of this post may have been influenced by my 'West Country Logic'.