Take for example json: Go's stdlib json implementation is wildly slow. It has to be implemented in reflection, it can't use any code-generation anything, and for backwards compatibility it can never trim any feature, no matter whether it has a performance implication for all users of the package. It's well known to be a slow implementation.
In java, json isn't part of the stdlib. There's jackson if you want one flavor of fast json, jetty ships its own json serializer, etc. This means that if you pick up a new java codebase written in a different framework, it's possible you have to learn the right pattern to json serialize things.
By nature of these third party libraries in java being out of the stdlib, they've been able to grow and become more performant and compete with each other.
Go sits on the opposite side. The stdlib http implementation is a little slow. The stdlib json implementation is incredibly slow. No one wants to build these features outside of the stdlib though, and people would rather every codebase have the same looking go code, even if that means the actual code itself is slower.