Document FFT performance problem and fine tune it's scheduling

4 jobs from pure_fork_join in 3 minutes 41 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #3128
build_cmake

00:42

 
  Test
passed #3129
run_tests

00:41

 
  Sanitizer
passed #3131
run_address_sanitizer

01:20

passed #3130
run_thread_sanitizer

00:57