LogFormat

%T - time spent to serve request in seconds
LogFormat "%T %h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-agent}i\"" time_format

# use in virtual host file
CustomLog /var/log/apache2/sitename_time.log time_format

Find requests that take a lot of time

cat last_logs | awk '{print $9,$1}' | sort -k1

Real time monitor of requests that take more than one second.

tail -f sitename_time.log  | awk '{if ($1 > 1) print ;}'

Read other log formats for apache2.