An inline slug contains a chimera, which is an `<img>` tag whose `src` attribute is a data URI. The image represented by this data URI is a 1x1px transparent GIF, and is never actually rendered because we set `style="width: 0; height: 0;"`. IIRC it's there because Firefox misbehaves when `<img>`s don't have an `src` that resolves to a real image.
In Chrome, however, this is terribly slow. Cloning `<img>` nodes with this `src`, or parsing HTML containing an `<img>` with such an `src`, or calling `imgNode.src = 'data:.....';` are all slow operations.
In `setupSlugs`, we create a single slug node (a "template") that is then cloned multiple times. Commenting out the `src` attribute caused the time spent cloning the slug template to decrease from 127ms to 4ms in my test case (the Barack Obama page). These images also end up being cloned by comparison code in `renderContents` (see also T91244), where time spent cloning decreased from 61ms to 4ms.
We should figure out if there's a way to not need to set the `src` of these `<img>` tags, at least in Chrome; and we should figure out if this is also slow in Firefox.