only -9%

name       old time/op  new time/op  delta
Iterate-4  17.4µs ± 0%  17.5µs ± 0%    ~     (p=0.234 n=10+9)
FsTail-4    278µs ± 1%   252µs ± 0%  -9.12%  (p=0.000 n=10+10)

because there are many IO direction misses.