"Minification" is basically removing all the human-readable-but-unnecessary-for-machines white spaces and line breaks.
This at BEST saves a few KB
But a few KB isn't going to slow your site down.
It's all the back-and-forth connecting to servers and external assets that cause slowdowns.
What's the difference between 90 KB unminified and 70 KB minified?
NOTHING. There is zero noticeable slowdown.
What ends up happening is that this gets cached/saved/stored and acts as a third/fourth/fifth layer of caching and people start complaining about "my changes aren't being reflected".
It literally just offers no benefit and causes problems.
It also prevents us from being able to troubleshoot, because instead of html/css being human-readable by us humans, it's all jumbled onto a single incomprehensible line.
Note: this also applies to autoptimize.