Well, the assembler looks the same to me too, but I can confirm different runtimes on two different machines (fast is always faster here).

  $ ./main.test -test.bench . -test.cpuprofile=test.profile
  goos: linux
  goarch: amd64
  BenchmarkInLoop-8       2000000000               0.42 ns/op
  BenchmarkOutLoop-8      2000000000               0.47 ns/op
  PASS

  (pprof) disasm fast 
  Total: 1.86s
  ROUTINE ======================== command-line-arguments.fast
       990ms      990ms (flat, cum) 53.23% of Total
           .          .     4ed7b0: MOVQ 0x8(SP), AX                             ;mem_test.go:16
           .          .     4ed7b5: XORL CX, CX
           .          .     4ed7b7: MOVQ CX, DX
           .          .     4ed7ba: JMP 0x4ed7c6                             ;mem_test.go:19
       650ms      650ms     4ed7bc: LEAQ 0x1(CX), BX                             ;command-line-arguments.fast mem_test.go:19
           .          .     4ed7c0: MOVQ CX, DX                               ;mem_test.go:19
        10ms       10ms     4ed7c3: MOVQ BX, CX                               ;command-line-arguments.fast mem_test.go:19
       330ms      330ms     4ed7c6: CMPQ AX, CX
           .          .     4ed7c9: JL 0x4ed7bc                               ;mem_test.go:19
           .          .     4ed7cb: MOVQ DX, 0x10(SP)                           ;mem_test.go:23
  (pprof) disasm slow
  Total: 1.86s
  ROUTINE ======================== command-line-arguments.slow
       870ms      870ms (flat, cum) 46.77% of Total
           .          .     4ed780: MOVQ 0x8(SP), AX                             ;mem_test.go:7
           .          .     4ed785: XORL CX, CX
           .          .     4ed787: MOVQ CX, DX
           .          .     4ed78a: JMP 0x4ed796                             ;mem_test.go:9
       560ms      560ms     4ed78c: LEAQ 0x1(CX), BX                             ;command-line-arguments.slow mem_test.go:9
           .          .     4ed790: MOVQ CX, DX                               ;mem_test.go:9
        20ms       20ms     4ed793: MOVQ BX, CX                               ;command-line-arguments.slow mem_test.go:9
       290ms      290ms     4ed796: CMPQ AX, CX
           .          .     4ed799: JL 0x4ed78c                               ;mem_test.go:9
           .          .     4ed79b: MOVQ DX, 0x10(SP)                           ;mem_test.go:13