I haven't tested this since then, but this wasn't the case in 2014[1]. I haven't noticed these issues since then, but back then it cost hundreds of microseconds -- 8 orders of magnitude more than "a few ns".

EDIT: I just re-did the relevant tests on Go 1.11.5. It's significantly better than in 2014 but it still costs between ~20us and ~50us ("only" 4 orders of magnitude more than "a few ns").

   goos: linux
   goarch: amd64
   BenchmarkPut-8             50000             27502 ns/op
   BenchmarkPutDefer-8        30000             46774 ns/op
   BenchmarkGet-8             50000             29812 ns/op
   BenchmarkGetDefer-8        20000             89701 ns/op
   PASS

[1]: https://lk4d4.darth.io/posts/defer/