If you compile in release mode, there is no effect because all the System.Diagnostics.WriteLine code is removed by the compiler-- no perf cost, no diagnostic benefits (in production) ref System.Diagnostics.Debug.WriteLine in production code
If you use TraceSource it's a different story, especially if you leave in the default trace listener, which is rather expensive. For production trace, you want something that you can turn on and off, usually via a config file so you only pay the perf cost for the duration of the diagnostic checks you are running on production.