That's even what they show in the chrome/firefox store images, it doesn't make much sense to me that it adds several seconds to Firefox startup time
Am I missing something ?
Be skeptical of claims which come with no technical details whatsoever to help you reproduce it?
It loads quite fast on Firefox, see https://github.com/gorhill/uBlock/wiki/Launch-and-filter-lis... -- see "Setup time" graph, typically less than half a second on Firefox.
That is the purpose of the built-in logger[1]. By default uBO is not setup to block other extensions' network requests, or those made by the browser itself, one has to configure it explicitly for this to happen[2].
1. https://github.com/gorhill/uBlock/wiki/The-logger
2. https://github.com/gorhill/uBlock/wiki/Behind-the-scene-netw...
Well, not all my addons - turns out though my Scriptish has been installed since Firefox 3 (now at 45), it wouldn't reinstall after removal for compatibility problems. Not sure why that wasn't automatically detected, and that may have been the performance issue