Overall, about the same in my experience. Where you'll notice a difference is older devices. Lag shows up on Firefox forks then. But usually you can throw klar/focus on and do fine.

Ublock is way better at ad blocking, across the board. Not much gets through cromite, but you will get the oddball one sneaking by now and then. Plus, ublock is easier to tweak.

Now, I will say that cromite looks better on most of my devices. It makes better use of the screen in subtle ways like tabs on a tablet. Just little things I like better visually, which is subjective as fuck, but I've used both cromite and fennec in specific recently enough it might be of interest since you're asking a general question.

Now, I run mull most of the time, but I check out fennec every now and then just for poo and giggles. There's never any surface level differences between the two.

I think it fair to say that on tablets, bromite is my preference for opening links from other things. It does so faster, and the tabs make switching between things easier for me. But on a phone, they're essentially equal. Since firefox and its forks are a better choice for the health of the internet, I use it almost exclusively on phones; and it's maybe 50/50 on tablets.

EDIT: Looks like I'm wrong, Fennec is still maintained. Seems the difference is that Mull is more strict about privacy, which means more sites may not work.

Original comment:

I forget what it was, but I think Mull is the "better" Firefox flavor right now. Maybe Fennec is no longer maintained? Idk. Maybe it'll come up if you search it.

Cromite is different. It's based on chromium instead of Firefox. Mull (and maybe Fennec?) probably has better privacy overall. There's a comparison page somewhere out there.

Much better, as Cromite still uses the Chromium engine, which is again Google.