How to log total connection time in Apache

I'm currently logging "The time taken to serve the request, in microseconds" using the %D directive (as specified on https://httpd.apache.org/docs/2.4/mod/mod_log_config.html). However, this seems to log only the time used by Apache to create the response. Is there some way I can log the total time that the client is connected to the server? For example, Apache may create the page in 200 ms, but the client takes 2 seconds to download the page, because of a slow network. I'm interested in the 2 seconds here, since I want to measure the clients performance relative to this specific server.