I'm not an expert on DragonFly, but from what I know it's design differences aren't really that radical. It differs substantially from FreeBSD, but it's not like a fundamental re-imagining optimized for SMP. Performance is a bit better in some places, and about the same in others.

I do think DragonFly is neat and quite interesting, but I suspect it didn't gain as much momentum because the community behind BSD is already small and reluctant to fracture without stronger reasons. It's worth noting that this is not like spinning off a new Linux distro where you're ultimately just repackaging different variations of the same basic OS - DragonFly is effectively an entirely different OS since the kernel is significantly different.

Slightly out of date, but still relevant benchmarks: https://www.phoronix.com/scan.php?page=article&item=freebsd1...