APACHE customized Logformat : %D

Hi everyone, we are currently troubleshooting our platform, trying to find out the root cause of a huge TTFB delay on some random HTTP GET operations. For this, and also to improve our monitoring solution later on, i'd like to customize the Apache logformat and add %D to the list of info to retrieve. This will display the " The time taken to serve the request". I'd like to know if anyone already put that in prod and if there are any tricks to know or to take care of?