If your browser is designed from the ground up to support add-ons, but can't handle a couple of them without slowing to a crawl, your browser sucks. Don't blame the user for a problem with the software.
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.
