Shout out to the Go team on a job well done. I took the time to re-run some benchmarks, here are the results (both 1.9.7 and 1.13.5 re-run on same hardware & OS).
iMac:tmp robertengels$ benchcmp bench_1_9_7.txt bench_1_13_15.txt
benchmark old ns/op new ns/op delta
BenchmarkAddFixed-8 1.29 1.01 -21.71%
BenchmarkAddDecimal-8 352 249 -29.26%
BenchmarkAddBigInt-8 15.3 14.8 -3.27%
BenchmarkAddBigFloat-8 80.6 80.1 -0.62%
BenchmarkMulFixed-8 4.40 4.60 +4.55%
BenchmarkMulDecimal-8 71.8 73.8 +2.79%
BenchmarkMulBigInt-8 17.8 17.4 -2.25%
BenchmarkMulBigFloat-8 37.1 35.4 -4.58%
BenchmarkDivFixed-8 4.75 4.63 -2.53%
BenchmarkDivDecimal-8 882 769 -12.81%
BenchmarkDivBigInt-8 56.8 45.4 -20.07%
BenchmarkDivBigFloat-8 121 109 -9.92%
BenchmarkCmpFixed-8 1.02 0.50 -50.69%
BenchmarkCmpDecimal-8 8.42 8.03 -4.63%
BenchmarkCmpBigInt-8 5.93 5.44 -8.26%
BenchmarkCmpBigFloat-8 5.11 5.63 +10.18%
BenchmarkStringFixed-8 60.1 58.6 -2.50%
BenchmarkStringNFixed-8 59.0 57.2 -3.05%
BenchmarkStringDecimal-8 259 214 -17.37%
BenchmarkStringBigInt-8 164 126 -23.17%
BenchmarkStringBigFloat-8 465 433 -6.88%
BenchmarkWriteTo-8 45.1 38.4 -14.86%