They don't! At least, not for me.
See my answer.
According to last year's Phoronix benchmarks, applications on FreeBSD mostly run slower than on Debian (including Stockfish chess engine, Node.js, FLAC encoding and other computational tasks).
Phoronix article itself attributes some of the performance differences to use of Clang instead of GCC compiler. Some other opinions say that use of ZFS makes FreeBSD slower, as ZFS is inherently slower than ext4.
But even purely computational tasks on FreeBSD compiled with GCC8 performed slower than on Linux.
What is the cause of that? Is it inherent to differences between FreeBSD and Linux kernels, might it be caused by worse quality of drivers or is there some other reason?
P.S. To make it more specific, here is a fairly simple purely computational program that runs slower on FreeBSD than on Linux according to Phoronix: m-queens 1.2. Compiled like this:
gcc -o m-queens.bin main.c -O2 -march=native -mtune=native -std=c99 -fopenmp
Since this a multithreaded task that was run on two 20 core CPUs, I suspect the performance difference boils down to how well OS handles multiple threads.
P.P.P.S. After installing FreeBSD (TrueOS) on my 6-core desktop alongside Ubuntu and trying to run the queens benchmark myself, I didn't notice any significant difference in multithreaded performance. While Phoronix claims that it ran 39% slower on FreeBSD, in my tests it was only 3.7% slower, which could be attributed to slight difference in compiler version (gcc 7.4 on TrueOS, gcc 7.2 on Ubuntu). I will test more later.