I am talking practical real world speed vs internet speeds. They may call it a 100mb connection but it could be a gazillion and if I’m downloading a movie and it says it’s coming down at 12mb/sec as far as I’m concerned it’s 12mb.
But, your internet is coming in over a shared bandwidth connection. That connection might support 1000mb, but you won't actually get that. You will get your "shared part" of it.
You stated that you do get much faster connection by connecting hard line directly to your incoming internet line. The first place I would look for a problem would be your repeater (extender) and then your wifi access point.
I can tell you that my internet connection is from a cellular router that usually only gets about 20mb up/down at max. (I can't get hardwire out here.) My house wifi is 2.4 GHz and I sometimes have as many as four video streams at once with no problems. But, my equipment is not cheap.