The preallocated memory size is comparable to bytes.Buffer bootstraping
array length (bytes.Buffer was used before rewrite to strings.Builder).
Without preallocation, encodeWord does more than one allocation for
almost any possible input.
The regression happens because bytes.Buffer did a 80-bytes allocation
at the beginning of encodeWord while strings.Builder did several
smaller allocations (started with cap=0).
Comparison with reported regression:
name old time/op new time/op delta
QEncodeWord-4 781ns ± 1% 593ns ± 1% -24.08% (p=0.008 n=5+5)
name old alloc/op new alloc/op delta
QEncodeWord-4 152B ± 0% 80B ± 0% -47.37% (p=0.008 n=5+5)
name old allocs/op new allocs/op delta
QEncodeWord-4 5.00 ± 0% 2.00 ± 0% -60.00% (p=0.008 n=5+5)
Comparison with buffer solution (like before strings.Builder, but
without sync pool for buffer re-using):
name old time/op new time/op delta
QEncodeWord-4 595ns ± 1% 593ns ± 1% ~ (p=0.460 n=5+5)
name old alloc/op new alloc/op delta
QEncodeWord-4 160B ± 0% 80B ± 0% -50.00% (p=0.008 n=5+5)
name old allocs/op new allocs/op delta
QEncodeWord-4 2.00 ± 0% 2.00 ± 0% ~ (all equal)