What affect will enabling the response time (%D) LogFormat directive dress in apache performance?
The response time it's time come to serve a request in microseconds.
Response time could be enabled like so:
#LogFormat "%h %l %u %t \"%r\" %>s %b" # Default LogFormat LogFormat "%h %l %u %t \"%r\" %>s %b %D" # LogFormat including response time
I've written something for monitoring and examining after sales script performance, apache-response-time, which requires response time data. I must warn potential customers how enabling this directive will affect apache server performance.
Most likely not worth thinking about when it comes to HTTP reactions -- just the overhead of just one addl gettimeofday() call (along with a couple of function calls) so something around the order of the couple of microseconds. Apache already notes exactly the same factor once the request arrives.
http://world wide web.atl.lmco.com/projects/QoS/POSIX_html/index.html lists some old microbenchmarks of gettimeofday() itself.