I used ack for a while, switched to ag (I don’t remember why? FOTM maybe?), and finally ended up with ripgrep. If you haven’t tried ripgrep you definitely should. It has almost completely replaced gnu grep for me.
I did the same but stayed with ag. For me, like with its contemporary find replacement fd, it's the UX that provides the most benefit.
The speed benefit isn't really a huge factor; working how I'd expect, omitting ignored files, being able to specify file extensions to search, plus simple editor integration. Amazing.
I keep meaning to replace find, but never get around to it. I will check out fd. Ty.