You didn't answer the other questions: did you check how much memory 2.4 used for the same sites (the same database and templates)? How many templates and how big they are?
Regarding the memory:
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND ... apache 13533 0.8 2.8 27384 14340 ? S 09:32 3:06 \_ /usr/sbin/httpd ... apache 4626 9.3 1.5 116076 7724 ? Ssl 08:11 41:25 /usr/local/campsite/bin/campsite_server -i campsite
What you saw on the web interface is the virtual memory size which is the 'virtual' memory the process uses. Why virtual? Because it sums up the memory used by all libraries accessed by the process. But these libraries are shared by all processes that use them so this IS NOT a reliable way of measuring the process memory. The real memory used by the process is RSS: 14340 for apache and 7724 for
campsite. Also, in case of a big site with many templates, many topics the template engine uses more memory to cache them.