Will Log writing in text file affect performance of a website?

Will Log writing in text file affect performance of a website?

I am have a asp website.I am writing log files to get total time span for
the page.Is writing log in txt files affect the performance of my
program?I am logging this in events like OnUnload.
private DateTime _startTime;
private DateTime _endTime;
string _page;
_endTime = CommonFunctions.getTodayDateTime();
TimeSpan span = _endTime - _startTime;
WriteLog(string.Format("The total time span for the '{0}' is {1}ms",
_page, span.TotalMilliseconds));