Startup.cs

using AppFunc = Func<IDictionary<string, object>, Task>;

public class Startup
{
    public void Configuration(IAppBuilder app)
    {
        app.Use(new Func<AppFunc, AppFunc>(next => async env =>
        {
            var stopwatch = Stopwatch.StartNew();
            
            await next.Invoke(env);
            
            stopwatch.Stop();
            
            // Write to log or something...
        }));
    }
}