looks like slice modification inside loop is not well optimized yet.

name       old time/op  new time/op  delta
PyQuote-4   894ns ± 0%   861ns ± 0%  -3.78%  (p=0.008 n=5+5)

name       old time/op    new time/op    delta
PyQuote-4     950ns ± 0%     894ns ± 0%  -5.89%  (p=0.008 n=5+5)

name       old alloc/op   new alloc/op   delta
PyQuote-4     0.00B          0.00B         ~     (all equal)

name       old allocs/op  new allocs/op  delta
PyQuote-4      0.00           0.00         ~     (all equal)

name       old time/op    new time/op    delta
PyQuote-4     976ns ± 0%     950ns ± 0%  -2.64%  (p=0.008 n=5+5)

name       old alloc/op   new alloc/op   delta
PyQuote-4     0.00B          0.00B         ~     (all equal)

name       old allocs/op  new allocs/op  delta
PyQuote-4      0.00           0.00         ~     (all equal)

it is annoying to see this function high in fstail profiles taking up to 10%

name       old time/op    new time/op    delta
PyQuote-4    1.33µs ± 1%    0.98µs ± 0%  -26.77%  (p=0.008 n=5+5)

name       old alloc/op   new alloc/op   delta
PyQuote-4     0.00B          0.00B          ~     (all equal)

name       old allocs/op  new allocs/op  delta
PyQuote-4      0.00           0.00          ~     (all equal)

name      old time/op    new time/op    delta
FsTail-4     166µs ± 0%     107µs ± 1%  -35.54%  (p=0.008 n=5+5)

name      old alloc/op   new alloc/op   delta
FsTail-4    28.1kB ± 0%    12.6kB ± 0%  -54.95%  (p=0.008 n=5+5)

name      old allocs/op  new allocs/op  delta
FsTail-4       671 ± 0%        78 ± 0%  -88.38%  (p=0.008 n=5+5)

on non testdata/1.fs

name        old time/op    new time/op    delta
ZodbDump-4     313ms ± 1%      98ms ± 0%  -68.81%  (p=0.008 n=5+5)

name        old alloc/op   new alloc/op   delta
ZodbDump-4    75.0MB ± 0%     0.1MB ± 0%  -99.89%  (p=0.008 n=5+5)

name        old allocs/op  new allocs/op  delta
ZodbDump-4     2.16M ± 0%     0.00M ± 0%  -99.91%  (p=0.008 n=5+5)

name       old time/op    new time/op    delta
Iterate-4    17.4µs ± 0%    13.1µs ± 1%  -24.65%  (p=0.000 n=10+9)

name       old alloc/op   new alloc/op   delta
Iterate-4    17.2kB ± 0%    10.6kB ± 0%  -38.29%  (p=0.000 n=10+10)

name       old allocs/op  new allocs/op  delta
Iterate-4       164 ± 0%        24 ± 0%  -85.37%  (p=0.000 n=10+10)

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.