Before ever venturing in performance comparison we need to define where, when, under what circumstances and for what purpose. Usually benchs are carried out on powerful modern workstation and servers, and focus on compiling databases/languages, I/O scheduling, garbage collection, executing shell/java/go scripts, simulations, audio/video encoding, rendering, packet filtering, write speed, backup, extracting archives.
Benchs usually do not focus on low-end hardware, laptops, tablets, embedded devices, gaming, graphic designing, multimedia streaming, text processing, download speed, etc...
I'm under the impression that DragonFly's performance benefits mostly apply to high-end hardware and that the system is not optimized for consumer-grade laptops. In my experience, on a low-end laptop equipped with a Intel Celeron, a small rotational HDD, 3 GB RAM, HAMMER (without swapcache) was a battery killer and would rather slow down your PC compared to other BSDs.
You also have to keep in mind that other OSs, often with a larger manpower and more conspicuous funds at hand, target performance as one of their primary goal too. This includes Linux, Solaris and FreeBSD. Overall, Linux at the current state is the top one challenger in the performance race. Solaris may still beat Linux when it comes to vertical scalability. FreeBSD lags behind a little bit, but again, it's TCP/IP stack is said to still perform slightly better than Linux', (referring to PF+ALTQ vs Netfilter). In spite of its focus on performance, DragonFly is not necessary a winner here: all benchmarks (yeah, Phoronix) reveal it more or less keeps up with FreeBSD, generally performing a little worse than the latter in more than one scenario.
A completely different argumentation involves performance on low-end and legacy hardware, or embedded devices. Here it's better to opt for a lightweight solution optimized to run under significant resource restraints, while taking the most of out them. NetBSD runs well on 8MB RAM, on limited CPUs like Amigas' m68k, and flies on older hardware. It's tuned to be essential, minimalist and as little resource-hungry as possible. Linux distributions specifically tailored for those scenario will fare just as well. Yet, even here our talk is pretty much relative and debatable. if we move onto 8bit CPUs, e.g. Z80, 8080, both Linux and NetBSD won't be even able to boot on them.
To sum up, if you want performance on BSD, my recommendation is:
With 8GB+ DR3/4 RAM, 3.x+ GHz CPU / 4+ cores, 256 GB+ storage, choose FreeBSD on ZFS or DragonflyBSD on HAMMER2
with everything below those specs, my personal choice goes to either NetBSD or Slackware (or other lightweight Linux distro, e.g. Void); FreeBSD/OpenBSD on UFS always representing a solid alternative choice.
