"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?

(edit3 - 10-19-2020) Were you searching a folder or a single engine when you observed the problem last? I've uploaded v1.27.5 which fixed the same problem v1.27.4 did, but when searching a folder as well.

(edit 2) Thanks for the reply. I thought the bug fixed in 1.27.4 would take care of your issue, but there must be another bug. I've tried all the engines you mentioned, but I'm getting good search results using the context menu now. Could you open ContextSearch options -> Search Engines and tell me if you see an orange (FF) next to those engines you're having issues with? If not, could you edit one of them and paste the template / method / etc info? It could help narrow down the problem.

(edit) Nevermind, version 1.27.4 going up. Context menu was not working with Firefox engines

Would you mind opening a ticket at the support site or editing your reply here? Some users seem to be experiencing problems with the latest builds and I could use some input on what exactly is going wrong. I tried adding Yandex to my install and had no problems, and troubleshooting is difficult when I can't replicate the bug.

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