They're presumably gaining the other non-performance advantages of Rust over C++ (i.e. the borrow checker etc.) though, so safer and faster is a win. As you say though, they seem to focus on performance but that might just be the audience they see themselves talking to.
I think "Chrome but faster" would convince a mainstream audience pretty well, even though they might not be aware that they're not going to notice the milliseconds.
if you're moving from Firefox to brave you do notice the speed increase. FF on my work laptop (i5, 16GB of ram) and my s8 is significantly slower that brave.
It's been a gift to me, since I really just can't help but install extension after extension and FF is just not good at that in my experience (plus the selection is narrower but I don't put that on FF). Another thing is that Brave has the Chrome url search which I've come to realize is actually pretty central to me, I just can't stand the 'prepending a letter' type search. Overall it's the closest I've felt to chrome with the stated goal of addressing the the primary reason I feel uncomfortable with chrome, so I'm happy for now.