"That's a self-fulfilling issue, though."
"If no one is using Gecko why does Gecko have reason to improve performance."
"If they had good performance engineers it wouldn't matter if they picked the "worst performing", because they could work to improve it and prove they had good performance engineers."
"Fast" is the second priority listed in Brave's webpage ("Secure, Fast, & Private"), so it is very easy to assume that it was one of their reasons for making a fork of a browser in the first place. It is more than fair to criticize them for taking the ("paved with bad intentions") easy road in that stated priority rather than accepting the true engineering challenge of doing that work on the underdog engine in current last place. (The basic through line of lamenting that they didn't use Gecko: If they had succeeded in that challenge, they would have won a lot more acclaim and done a lot more overall good for the web than anything they have done in forking Blink.)