Note that Nim standard library focuses on maintenance cost as the Nim team is small. When raw performance is needed, Nim gives us the tools to reach for it.
For instance at Status we use our own json serialization/deserialization library: https://github.com/status-im/nim-serialization and even Araq, Nim language creator, has his own JSON library: https://github.com/Araq/packedjson
This allows Nim to be in the top10 of json parsing in TechEmpower: http://www.techempower.com/benchmarks/#section=data-r18&hw=p...
Disclaimer: Status is the main sponsor behind Nim https://our.status.im/status-partners-with-the-team-behind-t... / https://nim-lang.org/blog/2018/08/07/nim-partners-with-statu...
Status is committed to Nim but we do have a pretty diverse stack (i.e. most of our code is in Clojurescript and we have part of the codebase in Go that we are migrating to Nim at the moment) and teams work very independently from each other, so other teams are sharing back their experiences with Nim.