(edit)
I took another look at how Speedometer 2.0 works and why the slowdown. It's not actually a content script loading issue. The page simulates a ton of click events. Every click event triggers a very light "menu close" event and method. When the addon gets flooded with hundreds of simulated click events per second, it bogs down the page. It's nothing you'd experience in a real-world situation. The Speedometer rating flat-out misleading.

This addon injects content scripts into iframes to maintain most if not all function when searching text inside iframes.

When testing using Speedometer 2.0, content scripts are being loaded into the iframe every refresh. If that iframe refreshes 480 times, or whatever the actual count is, it adds up.

For reference, I ran tests while injecting this addon's code into the iframe, or not -

with iframe injection: 29.41 runs/min
without iframe injection: 63.66 runs/min