You don't need to do "number crunching"; the performance differences are very real even for simple scripts on modern systems. Launching a process is comparatively expensive (read binary from disk, run linker, allocate new process space in kernel, etc.) whereas a builtin is just "if progname == "[" { .. }". And when you do something like "loop over a few thousand files" the difference really adds up.

Consider this example:

  for i in $(seq 1 10); do
      for f in /*; do
          if [ $f = /etc ]; then
              : # Do nothing.
          fi
      done
  done

I have 23 entries in /, so this will run [ 230 times – not a crazy number.

  % time sh test
  sh test  0.00s user 0.01s system 91% cpu 0.009 total

bash, dash, zsh: they all have roughly the same performance: "fast enough to be practically instantaneous".

But if I replace [ with /bin/[:

  % time sh test
  sh test  0.11s user 0.38s system 96% cpu 0.509 total

Half a second! That's tons slower! And you can keep "fast enough to be practically instantaneous" for thousands of files with the built-in [, whereas it will take many seconds with /bin/[.

(Aside: if I statically link [ it's about 0.4 seconds).