Add Performance notes on using ONLY fork_join tasks.

4 jobs from master in 3 minutes 40 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #2924
build_cmake

00:43

 
  Test
passed #2925
run_tests

00:41

 
  Sanitizer
passed #2927
run_address_sanitizer

01:19

passed #2926
run_thread_sanitizer

00:55