(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) v1.21+ has some clipboard features in the Advanced menu
I'll give it a shot

> This addon is great but is it possible to run all search engines in folder at the same time?

When using the popup menu, yeah ( context menu folders have no usable click events ). Clicking the middle mouse button on a folder will search all engines in the folder with the current search terms and send the results to background pages. You can change this behavior in Options -> Quick Menu -> Search Actions -> Folders column.

> 2nd thing is hotkey assigned to search engine. Is it possible to make not a single letter but e.g. 2-letter / 3-letter phrase?

Not currently. Be aware, the hotkeys only work with the quick menu opened. If you're looking to select text and press a key to immediately perform a search, you might want to open the qm using the keyboard method ( using CTRL for instance ) and then you can press your preferred search engine hotkey to execute a search. If you're looking for more functionality, consider opening a thread on the github support site.