Consider using Grisu3 + Dragon4 for double.ToString() conversion.

[mazong1123]

I saw we are using library function swprintf_s to convert a double value to string, which has performance issues that have been discussed in .NET Core. See https://github.com/dotnet/coreclr/issues/10390 and https://github.com/dotnet/coreclr/issues/10651

In .NET Core 2.0, there's a new implementation that combined Grisu3 + Dragon4 algorithms to significantly improved the performance (I should say probably 7x faster). See these two PRs: dotnet/coreclr#14646
dotnet/coreclr#12894

Replace swprintf_s with the new implementation could make the double conversion much more faster.

[Artoria2e5]

Or maybe the C Runtime people can be persuaded to just implement that in their library? That would be a bit idealistic, of course...