I'm surprised that you ask the contrary to what the rest of us want. But I will not be the one to judge you.
To the case. If I attend to what you explain, I'm not sure that someone has invented an app so that a smartphone, being connected to the internet, will go slower.
However, if you want to manage the data or not to pass, for that you must enter Settings / Connections / mobile data / data usage. And from there you manage the app that you do not want to connect and / or notify you when you exceed certain mobile data consumption.