Debug helper for Sitecore Renderings - based on http://www.letsdositecore.com/rendering-debug-information-in-sitecore-mvc-views/

PerformanceMeasurementProcessorBegin.cs

using System.Diagnostics;
using System.Web;
using Sitecore.Mvc.Pipelines.Response.RenderRendering;

namespace Website.Pipelines.RenderRendering
{
    public class PerformanceMeasurementProcessorBegin : RenderRenderingProcessor
    {
        public override void Process(RenderRenderingArgs args)
        {
#if DEBUG
            args.Writer.WriteLine();
            args.Writer.WriteLine("<!-- START: {0} -->", args.Rendering.Renderer);

            var sw = new Stopwatch();
            HttpContext.Current.Items["PerformanceMeasurementProcessor:sw"] = sw;
            sw.Start();
#endif
        }
    }
}

PerformanceMeasurementProcessorEnd.cs

using System.Diagnostics;
using System.Web;
using Sitecore.Diagnostics;
using Sitecore.Mvc.Pipelines.Response.RenderRendering;

namespace Website.Pipelines.RenderRendering
{
    public class PerformanceMeasurementProcessorEnd : RenderRenderingProcessor
    {
        public override void Process(RenderRenderingArgs args)
        {
#if DEBUG
            var sw = HttpContext.Current.Items["PerformanceMeasurementProcessor:sw"] as Stopwatch;
            if (sw == null)
            {
                Log.Warn("Stopwatch not found!", this);
                return;
            }

            sw.Stop();
            if (sw.ElapsedMilliseconds > 100)
                Log.Warn($"PERFORMANCE ALERT: {args.Rendering.Renderer} Time taken: {sw.ElapsedMilliseconds}ms", this);

            args.Writer.WriteLine();
            args.Writer.WriteLine($"<!-- END: {args.Rendering.Renderer} Time taken: {sw.ElapsedMilliseconds}ms -->");
#endif
        }
    }
}

z.PerformanceMeasurementProcessors.config

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>
    <pipelines>
      <mvc.renderRendering>
        <processor type="Website.Pipelines.RenderRendering.PerformanceMeasurementProcessorBegin, Website" patch:before="processor[@type='Sitecore.Mvc.Pipelines.Response.RenderRendering.ExecuteRenderer, Sitecore.Mvc']"/>
        <processor type="Website.Pipelines.RenderRendering.PerformanceMeasurementProcessorEnd, Website" patch:after="processor[@type='Sitecore.Mvc.Pipelines.Response.RenderRendering.ExecuteRenderer, Sitecore.Mvc']"/>
      </mvc.renderRendering>
    </pipelines>
  </sitecore>
</configuration>