I have a custom blank page loaded from SSD that has only a small amount of html5/pure js and it takes maybe 1000ms on an intel i5/3xxx. If I open a new tab without any other windows/tabs open it's very fast, maybe 100ms. A new private tab (doesn't load add-ons or my custom new tab) opens quickly but not as quickly as a fresh restart w/ no windows/tabs.
And then it was fixed. No reason to assume it still applies, unless you have new data.
Yes, having a great number of intrusive extensions can cause issues, but there is no need to jump to conclusions as these issues can be present without the user doing anything "wrong".
Why not all three? The browser implementation, user's choices, and the implementations of each individual add-on suck.
The browser didn't limit the resource use of greedy add-ons (or inform the user of conflicts between them), the add-ons were coded either to do too much, or to perform their function with suboptimal code, and the user is the one that chose that set of software to use together.