Phoronix has done a plethora of benchmarking tests highlighting performance differences between FreeBSD, Linux, TrueOS, DragonflyBSD. The author measures things like ZFS on FreeBSD vs ZFS on Linux and HAMMER2, networking performance, and others. Also, the Phoronix Test Suite has been updated to 8.6-M3. Check out the following links to look at the benchmark results.

Phoronix Test Suite 8.6-M3:

Fresh Linux vs. BSD CPU/System Benchmark Results Across Five Operating Systems:
FreeBSD ZFS vs. TrueOS ZoF vs. DragonFlyBSD HAMMER2 vs. ZFS On Linux Benchmarks:

Out-Of-The-Box 10GbE Network Benchmarks On Nine Linux Distributions Plus FreeBSD 12:

Windows Server 2019 vs. Linux vs. FreeBSD Gigabit & 10GbE Networking Performance: