GraphQL json parser and AllowSynchronousIO

Question

I'm starting to implement GraphQA in .NET with https://github.com/graphql-dotnet/server, I wrote a very simple sample application and while running Graphiql I got the following error:

fail: Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware[1]
      An unhandled exception has occurred while executing the request.
System.InvalidOperationException: Synchronous operations are disallowed. Call ReadAsync or set AllowSynchronousIO to true instead.
   at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpRequestStream.Read(Byte[] buffer, Int32 offset, Int32 count)
   at System.IO.StreamReader.ReadBuffer(Span`1 userBuffer, Boolean& readToUserBuffer)
   at System.IO.StreamReader.ReadSpan(Span`1 buffer)
   at System.IO.StreamReader.Read(Char[] buffer, Int32 index, Int32 count)
   at Newtonsoft.Json.JsonTextReader.ReadData(Boolean append, Int32 charsRequired)
   at Newtonsoft.Json.JsonTextReader.ParseValue()
   at Newtonsoft.Json.JsonReader.ReadForType(JsonContract contract, Boolean hasConverter)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize(JsonReader reader, Type objectType, Boolean checkAdditionalContent)
   at Newtonsoft.Json.JsonSerializer.DeserializeInternal(JsonReader reader, Type objectType)
   at Newtonsoft.Json.JsonSerializer.Deserialize[T](JsonReader reader)
   at GraphQL.Server.Transports.AspNetCore.GraphQLHttpMiddleware`1.Deserialize[T](Stream s)
   at GraphQL.Server.Transports.AspNetCore.GraphQLHttpMiddleware`1.InvokeAsync(HttpContext context)
   at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Cors.Infrastructure.CorsMiddleware.InvokeCore(HttpContext context, ICorsPolicyProvider corsPolicyProvider)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

I found a workaround here https://github.com/graphql-dotnet/graphql-dotnet/issues/1116 which states that the configuration for IIS/Krestel needs to allow Synchronous IO using the following:

services.Configure<IISServerOptions>(options =>
{
    options.AllowSynchronousIO = true;
});

I see that https://www.nuget.org/packages/GraphQL.Server.Transports.AspNetCore.NewtonsoftJson is still in alpha, so I'm guessing I have to use the build in JSON parser of .NET Core 3.1.

What are the implications of setting AllowSynchronousIO would it make the calls the server can handle slower?