With an old phone Firefox is indeed slow. Startup is slow, page loading is slow, even scrolling is sluggish. On Chromium browser the performance is a lot better if your phone is not that fast.

Why is Kiwi a terrible browser?

But Kiwi doesn’t open links from PWAs in default Browser, so no solution for me

On Firefox it works if you change the settings, but unfortunately i don’t know any Chromium browser with that feature.

It indeed works, but Firefox and derivatives are really slow on my phone :x

In settings there is “open links in apps”. If you change this links are indeed open in the default browser