.NET Core 3.0 introduced a json serializer in the System.Text.Json namespace to replace Newtonsoft.Json in high performance scenario. You'll find many benchmark on other website showing the performance gains when switching to S.T.Json.

By default, JSON in ASP.NET Core will not be formatted at all. By that I mean, the JSON is returned without any spacing or formatting, and in the cheapest way possible (from a “bytes-sent-over-the-wire” perspective).