I wouldn't worry, Go is not as fast as everyone believes it to be, particularly for tasks like these, unlike C# - there's a reason Nethermind Ethereum client uses pure C# without ever having to touch C or ASM (the performance critical parts of Geth, another Ethereum client that uses Go, are implemented in C and Go's "asm" instead).
Chose to fix that, but won't delete the submissions that used it. Thanks!
I used a classic cheat code to get a valid entry and a bit higher :D
jodrellblank/710260141000+idkfa
jodrellblank/uhxbvhbaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
00FACADE F2036CC41D004F2EB0F35B079C589D724D5A49BD796363C9A13C27D8
Bug: The site says "nonce: 1-64 characters from Base64 (a-zA-Z0-9+/)" but it accepts "=" as well.
Reading the other answers about hand optimised CUDA and parallel Go and Rust, that's probably as high as I'll get.