Performance has regressed with v0.21.0

[StephenWakely]

This is v0.19.0 against v0.21.0.

Can this be improved:

     Running benches/bench.rs (target/release/deps/bench-d903518f8b658a05)
Gnuplot not found, using plotters backend
RFC5424/with_structured_data/177
                        time:   [1305.0943 cycles 1307.0348 cycles 1309.2186 cycles]
                        thrpt:  [7.3967 cpb 7.3844 cpb 7.3734 cpb]
                 change:
                        time:   [+2.9012% +3.1793% +3.4132%] (p = 0.00 < 0.05)
                        thrpt:  [-3.3005% -3.0814% -2.8194%]
                        Performance has regressed.
Found 4 outliers among 100 measurements (4.00%)
  3 (3.00%) high mild
  1 (1.00%) high severe
RFC5424/with_structured_data_long_message/1893
                        time:   [830.6381 cycles 834.5181 cycles 840.1273 cycles]
                        thrpt:  [0.4438 cpb 0.4408 cpb 0.4388 cpb]
                 change:
                        time:   [+8.3298% +9.2016% +9.8649%] (p = 0.00 < 0.05)
                        thrpt:  [-8.9791% -8.4262% -7.6893%]
                        Performance has regressed.
Found 6 outliers among 100 measurements (6.00%)
  1 (1.00%) low mild
  5 (5.00%) high severe
RFC5424/with_structured_data_long_message/1960
                        time:   [1328.7230 cycles 1329.8759 cycles 1331.1333 cycles]
                        thrpt:  [0.6791 cpb 0.6785 cpb 0.6779 cpb]
                 change:
                        time:   [+2.5727% +3.1048% +3.4679%] (p = 0.00 < 0.05)
                        thrpt:  [-3.3517% -3.0113% -2.5082%]
                        Performance has regressed.
Found 7 outliers among 100 measurements (7.00%)
  5 (5.00%) high mild
  2 (2.00%) high severe
RFC5424/without_structured_data/110
                        time:   [814.9914 cycles 815.9531 cycles 817.0249 cycles]
                        thrpt:  [7.4275 cpb 7.4178 cpb 7.4090 cpb]
                 change:
                        time:   [+5.9475% +6.4665% +6.8727%] (p = 0.00 < 0.05)
                        thrpt:  [-6.4307% -6.0737% -5.6136%]
                        Performance has regressed.
Found 10 outliers among 100 measurements (10.00%)
  2 (2.00%) low mild
  5 (5.00%) high mild
  3 (3.00%) high severe