Hi there!

Got the very same issue as you and some of my guildies on PS4/PC suffer from the same lag problem since a month at least.
We are on Mateus.
They are also on Videotron.
Plugged with a Lan cable doesn't change a thing.
Other online games play #1.
We called Videotron to check our connection and everything is perfectly fine.
Several people I've crossed paths with in donjons said that I wasn't the only one lagging.

Since yesterday, the launcher work erratically. Several parts of the login screen load randomly OR the "Connect" button doesn't work when pressed on. The only trick I've got is to restart the console until the page loads and finaly "works". I took me 20 minutes yesterday and this morning to get connected.

At first I thought that the Heavensward pre-order was the issue for the launcher but it's not the case

Hope it helps (at least knowing you're not the only one)