GeezerVison

                                                      confidence improvement accuracy (*)   (**)   (***)
fs/bench_fdatasyncSync.js n=10000 type='existing'            ***      6.39 %       ±1.85% ±2.48%  ±3.25%
fs/bench_fdatasyncSync.js n=10000 type='non-existing'        ***    116.11 %       ±6.89% ±9.21% ±12.08%

Be aware that when doing many comparisons the risk of a false-positive result increases.
In this case, there are 2 comparisons, you can thus expect the following amount of false-positive results:
  0.10 false positives, when considering a   5% risk acceptance (*, **, ***),
  0.02 false positives, when considering a   1% risk acceptance (**, ***),
  0.00 false positives, when considering a 0.1% risk acceptance (***)