For your example, my output is
# $ nim c t.nim
$ ./t
Benchmark: fib5()
Collected 241 samples
Warning: Found 6 mild and 15 extreme outliers in the time measurements
Warning: Found 8 mild and 10 extreme outliers in the cycles measurements
Time
Mean: 251.7599ns (249.2240ns .. 255.4843ns)
Std: 24.0680ns (7.8357ns .. 37.5615ns)
Slope: 249.0758ns (248.7008ns .. 249.5158ns)
r^2: 1.0000 (1.0000 .. 1.0000)
Cycles
Mean: 647cycles (642cycles .. 654cycles)
Std: 46cycles (15cycles .. 72cycles)
Slope: 645cycles (644cycles .. 646cycles)
r^2: 1.0000 (1.0000 .. 1.0000)
Run
Watch for relation of cycles to ns, it is in the range 1..10 as expected. For
your github page it seems to be approx 1000? Wrong scaling for me, or do i miss
something?