"The extension is good, but a message constantly pops up on a yellow background at the top of the screen stating that this add-on slows down the loading of web pages"

The only time I've seen real slowdown is during the marking of very short search results on a large page or using the Find bar to search across all tabs. When are you seeing the slowdown?

That's unfortunate. If you want to try and fix the issues you're having, open a thread at the support site and I'll troubleshoot with you.

(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

(edit v1.4.1)
Simplified UI for adding engines, defaults to directly adding engines to ContextSearch and prompts for installing OpenSearch engines via the API
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The api isn't 3rd party. It's mine written specifically for this add-on. It just happens to be hosted on google's app engine with a generic name.

(edit) It probably could stand to be simplified a bit for novice users. The complexity comes from attempting to integrate with Firefox's One-Click search engines. If I kept it simple with bookmark keywords, the search engines wouldn't be added to the OpenSearch engines in the search toolbar. Adding OpenSearch engines requires an external URL, so the api is necessary. I'll look into adding a simple/advanced option that bypasses all OpenSearch integration and just adds to ContextSearch

(edit 5-11)
Sorry I didn't see your edit sooner.

If you open ContextSearch options, scroll down to Opening and Closing and you can choose a method to open the quick menu.

Automatic (popup opens after selecting text)
Mouse hold (hold either the left or right button until popup appears)
Mouse click (right-click to open )
Keyboad button (press a keyboard button of your choice to open)
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Does it not work on any website or just addons.mozilla.org? Mozilla disables certain add-on features on addons.mozilla.org for what I assume are security reasons. Using default settings, it should work by highlighting text and holding right-click until the menu appears ~0.25seconds.