I don't have any issues with it. NoScript and uBlock Origin are working fine for me.

That's pretty harsh. It works fine for me. But even if it didn't, I'd still use it just for uBlock Origin.

I'm running it on a device with a Qualcomm SM8635 Snapdragon 8s Gen 3 chipset, and it just crawls. The UI is very unresponsive, and page load times are terrible.

For youtube background play Brave is much better.

Yes. The web, and especially the web on mobile, is unusable without an ad blocker.

Edge canary runs on android with full extension support?

Yep. Enable extensions in edge://flags/. Then you can use ublock origin. You can install any crx file extension if you enable developer mode.

Strange, I am running it on a Snapdragon 8 Gen 2 (Z Fold 5), and it's totally fine for me. (If anything, it's a little too good at staying in the background; if you have private tabs open it insists on persisting in memory.)

Not saying your issues aren't real, but rather maybe there's another app or your manufacturer's flavor of Android that's causing the issue (like those aggressive background killers).

As for Edge, I used to be a big fan, but when they finally introduced history and tab syncing in 2021, it didn't have E2EE, and it still doesn't, which I find inexcusable. All the other major browser vendors offer it, even Google (though you have to opt in).

I have a OnePlus 5T from 2017 with a Snapdragon 835 processor and find that Firefox works perfectly well on it. I also have a device with a Snapdragon 8cx Gen 3 processor and Windows 11 on ARM. Firefox runs smoothly on that too.