RestClient.Execute() is extremely slow compared to RestClient.ExecuteAsync().Result

Describe the bug
RestClient.Execute() is extremely slow compared to RestClient.ExecuteAsync().Result, when called in a parallel setting, even though both are synchronous.

To Reproduce

private class CustomInterceptor : Interceptor
{
    public override ValueTask BeforeDeserialization ( RestResponse response, CancellationToken cancellationToken )
    {
        response.ContentType = ContentTypes.application_json;
        return ValueTask.CompletedTask;
    }
}

static void Test02 ()
{
    //
    const int total = 100;
    int width = ( int ) Math.Ceiling( Math.Log10( total ) );
    ( int, TimeSpan )[] elapsed = new ( int, TimeSpan )[total];
    //

    RestResponse    Execute    ( RestClient c, string resource, Method method, Action<RestRequest> callback )
    {
        //
        RestRequest     i;
        RestResponse    o;
        //

        if ( c == null )
            throw new ArgumentNullException( nameof( c ) );

        i = new RestRequest( resource, method );
        callback?.Invoke( i );
        i.Interceptors = [];
        i.Interceptors.Add( new CustomInterceptor() );
        o = c.Execute( i );
        return o;
    }

    void Test ( int i )
    {
        //
        Stopwatch watch;
        //

        static void Add ( RestRequest t )
        {
            t.AddHeader( HTTPHeaders.Content_Type, ContentTypes.application_json );
        }

        watch = Stopwatch.StartNew();
        using ( var c = new RestClient( some server ) )
            Execute( c, "", Method.Post, Add );

        watch.Stop();
        elapsed[i] = ( i, watch.Elapsed );
    }

    Parallel.For( 0, total, Test );

    ////

    Array.Sort( elapsed, ( x, y ) => y.Item2.CompareTo( x.Item2 ) );
    for ( int i = 0; i < total; i++ )
        Console.WriteLine( $@"{elapsed[i].Item1.ToString().PadLeft( width )} => {elapsed[i].Item2:mm\:ss\.fff}" );
}

If you use the above code, which uses c.Execute( i ), to communicate to a server, there will be very long delays (up to roughly 10 s). If you replace c.Execute( i ) with c.ExecuteAsync( i ).Result, the time is reduced to just roughly 1 s.

Expected behavior
Since both RestClient.Execute() and RestClient.ExecuteAsync().Result are synchronous, both should have the exact same performance.

Desktop

• OS: Windows 11
• .NET 8.0
• Version 114.0.0

Start by tracing the RestClient.Execute() and RestClient.ExecuteAsync() entry points and reproduce the timing difference with the provided Parallel.For example on .NET 8. Compare their synchronous execution paths under parallel use, then verify that equivalent calls have comparable performance and that the reported delay is resolved.