Please, someone explain to me why I can't level quick anymore?

In order to do the missions, you, and 2-3 others, need to be well geared, like the newest expansion, gear. The same is true for zones grinds that haven't had their experience cut-to-the-bone.

IOW, if you are a casual player, not in a busy time zone or a highly populated server, your ONLY choice for experience is Overseers.

That's not accurate--you can do the ToL missions in CoV gear, and you can level all the way to 120 killing mobs, you don't have to do the missions at all.

I bet you could do those missions with cov gear and cov spells. So long as you are able to follow emotes.