On my machine (nMP with dual FirePro D700’s) there’s a noticeable “stall” when Quantum navigates to a page. This isn’t there on Blink or WebKit. I’m not sure how to go about trying to get it looked at.
To determine whether this is the cause for your particular problem, create a fresh new macOS user account, log into it and see if you can replicate the slow-down there. If there's no slowdown, it's probably this bug.
To work around the problem, I created a temporary admin account and logged into it. I then renamed my current account, created a new one with the correct details, then transferred most of my stuff from the old account to the new one, including the full contents of Library. Last step, chown the new profile directory:
sudo chown -R youruser /Users/youruser