Dear Dolivo,
For 20 hens in the Summer heat, I would be happy with a girl turning out an egg every other day and a matron one every three days. 14 eggs for 20 hens in a heat wave is pretty good in my book. The first couple of years, even in the blazing heat, some hens will lay 9 days out of 10 and only a few every day for a long time. Lots of water, shady places, room to move around and alternative nesting spots ... all these things will help.
The variety makes a big difference, too. What varieties are you keeping? If it is a variety known for steady egg laying, then my prediction is that after a bit production will pick back up again, no worries, even with the heat, provided that there is space, shade, and so on. 15 to 18 down to 12 to 14 is such a small drop that I would not give it a thought. It could just be a pause after the first rush of eggs.
What about other stressing factors besides heat? Has a bully started disrupting the harmony of the flock? Has a dog been around barking at them? Any change in diet. Could be a hundred things. It might just be the new additions getting old enough to be treated as adults, resulting in some pecking order shifting. You will be getting a ton of eggs in no time, with the new girls coming on. It is all good.