Jalapenos are usually touted as being one of the most heat tolerant peppers. However, it seems that when the summer heat kicks in, I usually get only little 1-2 inch nubs from my jalapenos. On the other hand, Cubanelle's seem to tolerate high temperatures better and keep producing 4-5 inchers.

What is the best heat tolerant pepper you've grown?

Several but can't remember all the names.
Habanero is one and pepperoncini serrano Corno de Toro and so on.
Ghost pepper not heat tolerant at all

It doesn't get quite as hot here as where you are ( ), but it seems to me that most chinense peppers are heat resistant, as well as those Thai peppers, which seem to increase output, rather than be badly affected by heat. I usually have good results with jalapeños, and one I have had the best with is a hybrid, unfortunately, so it can't be saved - Jalafuego. I have to cover my jalapeños, up to around August 1st, due to pepper maggot flies, so the temp is even hotter in those mini "greenhouses", and other varieties are definitely slowed down, until uncovered.

Here's a photo from one season when I uncovered some Jalafuego plants, showing they had grown about 4' tall, and some of the peppers, and how long they got. I uncovered a few times, to harvest, and those south facing covered Earthboxes gets over 100° on many of those hotter days I have!
Jalafuego, uncovered 7-28 by pepperhead212, on Flickr

I also had some longhorns, fresnos, aleppos, and Hanoi markets growing to the tops of those cages I had over them that year. And probably others.
Pepper plants growing through the tops of the covers 7-28 by pepperhead212, on Flickr

It gets pretty hot here in Southern Colorado and jalapeños, cayennes and habaneros all do well. But I apply pine needle mulch around them.

Here in South East Texas, Poblanos, Aji Crystal, Cayenne, Anaheim, Tabasco, Jalapeños, Serrano, Pepperoncini all set fruit and do well in the height of summer here. Photos of peppers all around August 1st, 2022.

Do you think Jalafuego is really an F1 or do they just call it that for sales? I have seeds saved to F5 or F6 generation...I don't notice much difference; well, I don't notice ANY difference. They are still HOT for a jalapeno and still big and productive.

I saved Jalafuego seeds once, and started one, and it seemed totally different - I always try things like that once, to see.