Why does slow internet make my phone unusable?

[AC Question]

I've a Galaxy S5 and my wife a Galaxy S7 and every time we are using a bad internet connection we can't use our phones. By that I mean even launching the calculator can take ages.
I have to turn the internet off to make the phone usable again and that (opening config, turning wifi off ) also can takes ages.
Is it normal? I don't see way I can't use an app that doesn't require internet to work.
Is there a fix for that?
Thanks

 

[Rukbat]

First, if you pull the notification bar down, you should be able to turn internet off there. One pull, one tap.

Two - because the phone is probably spending most of its time trying to connect to the internet. If you're where there won't be any internet, just turn wifi off. (I automatically do that when I leave home. I have Tasker turning wifi off if the signal disappears for more than 5 seconds. That means I'm going out and won't have wifi.)