Devel : what kind of information devel_boot, devel_shutdown and php tweak describes?
devel_boot is the memory in use during hook_boot. It is the amount of memory Drupal is using just getting bootstrapped and ready
to run module and theme code.
php peak is the maximum amount of memory in use during your page request. It is provided by the PHP function
memory_get_peak_usage. This is probably the most important metric to consider.
devel_shutdown is the memory in use at the last possible moment it can be measured with code in a request. It is registered
via drupal_register_shutdown_function() which registers functions to be called once all other PHP code has finished executing.


Memory is important to monitor, but it doesn't seem like you are having issues with memory use. I can see from your screenshot that you are using 42MB of memory - that is nothing to be concerned about in most cases.

I would be more concerned about your slow cache queries (highlighted red in your screenshot). Try and figure out why they are taking so long to run.

There are a lot of resources on the web and drupal.org on improving performance. The biggest wins will come from caching and improving SQL performance - then you can start honing in on code profiling to figure out where your time is best spent.

Some resources that may help:


Performance articles on drupal.org
Tuning mysql
Presentation at Drupalcon London


Best of luck!