This turned to be very easy; for a start, I installed the xdebug package with:
yum install php-pecl-xdebug
then I edited the configuration file
xdebug.auto_trace = On
xdebug.show_mem_delta = On
Now I reloaded the page in the browser and got a shiny detailed report about memory consumption during the code execution in /tmp:
From that kind of report it is quite easy to spot big memory hoggers; in my case, I discovered about half of the peak memory usage (about 29Mb) is used for composing a single form control...