The change looks good, but I'm a little surprised it caused such regressions. It looks like CreateFromElement is also using this constructor but before that is checking ValueKind against the same three cases above. Those branches were enough to cause significant regressions? This must be a super hot path?

It's in the hot path for the lazy conversion ofJsonArray andJsonObject fromJsonElement backed to List and dictionary backed, which is what regressed by this change.

I wonder if theref readonly parameter in the removed method contributed to the regression somehow.

My guess is that it callselement.ValueKind twice, which resolves as follows

publicJsonValueKindValueKind=> TokenType.ToValueKind();`

privateJsonTokenTypeTokenType=> _parent?.GetJsonTokenType(_idx)?? JsonTokenType.None

internalJsonTokenTypeGetJsonTokenType(intindex){CheckNotDisposed();return_parsedData.GetJsonTokenType(index);}

internalJsonTokenTypeGetJsonTokenType(intindex){AssertValidIndex(index);uintunion=MemoryMarshal.Read<uint>(_data.AsSpan(index+NumberOfRowsOffset));return(JsonTokenType)(union>>28);}

plus a small cost to run the switch statement ininternal static JsonValueKind ToValueKind(this JsonTokenType tokenType)

Which is quite a lot of indirection. Possibly changingCreateFromElement to create a localJsonValueKind kind = element.ValueKind; to avoid running the above twice? But still maybe running it once is even a lot here.

Created a PR here#104108