I'm noticing a slow down in my internet too. I am aware of the effect that the number of devices can have, traffic at various times of the day, Wifi strength, etc., etc. but I am also aware that when I had Spectrum/Brighthouse, I had all the same devices connected with the same traffic patterns and never had the slow speeds that I have now.
In addition, when we signed up for AT&T wireless I was under the impression that my bill would be lower than it was with Sprint. It is not. It is about $20 a month more.
All in all, I am very disappointed with the whole package and am considering looking elsewhere.