Most stores do not have one slow plugin. They have twelve slightly careless ones, each adding a script, a query and a render-blocking call nobody measured.

The usual culprit is loading everywhere. An add-on that only matters on the cart page has no business running its javascript on the homepage, yet most do, because loading conditionally is more work than loading always.

The next is the database. A plugin that runs an uncached query on every page view is invisible in testing and fatal on sale day. Cache it, or scope it, or do not run it.

We audit for both before we add a line of our own. The fastest feature is the one that only wakes up when it is needed.