NT_Service Monitor has some limitations due to WMI connection. It uses lots of network resources and doesn't allow monitoring multiple services. To improve the WhatsUp Gold performances, we may use NRPE Monitor. You can find different tests and measures in the attached document and some NRPE agent explanations.

http://www.orsenna.org/ftp/companion/Forums/Improve_WUG_perf_with_NRPE.pdf