there's a number of benchmarks you can checkout online.
number crunching wise, Go is on par with Java and C#: https://benchmarksgame-team.pages.debian.net/benchmarksgame/...
for webdev work, the standard library might not be that fast, but it's fast enough, and if you need even more speed the fasthttp third party Implementation offers good speed: https://www.techempower.com/benchmarks/
https://benchmarksgame-team.pages.debian.net/benchmarksgame/...
Sort of supports my pulled out of thin air numbers (50-150%). In C/C++ you got those lovely SSE intrinsics ;) But look at the binary tree entry as well, that sort of x13 win for C++ really shows you how C++ lets you get sheer power with decent abstractions.
On the perf side: I'm curious to know where Go sits now relative to C assuming you elide away all heap allocations in your inner loops. In the early days Go's codegen was based on plan9's simple and simplistic C compiler's back end. Things have gotten much better since then and to my knowledge it's within striking distance of C. But I could be wrong.