Alright, more of a eli5 as I'm more folk knowledge than a scientist.

It's a narrower (more dense) wavelength.

If you think of signal, any signal, how close you are to it, the total power of that signal and the quality of your receiving gear are going to be your three major factors in "speed".

5g gains the ability to broadcast more waves iif you're close, at the expense of distance.

If you're looking to send communications further; wider (lower density) waves face less resistance. Just the same way you can seemingly get AM radio (bouncing off our atmosphere) anywhere vs FM radio (line of sight), each has a function.

You can find rural houses like mine, or the futures trades riding from the burbs to downtown with microwave (narrower than 5g) connections. They're pretty atmosphere resistant but require tuning to hit relays the size of about a soda can.

I don't think the longitudinal studies have been done on what frequencies over long periods of time produce negative results, the areas of spectrum we are working with have no real analogues in scope I'm aware of. Which is exactly why there's room to scaremonger over it.

Anecdotally I've worked a decade in an adjacent field and never heard of anyone contacting the plague.