1) For heavy backend operations like generating reports, don't generate the screen on the init hook. Schedule a cron job and generate it periodically ... if the administrator needs it right away, give them a refresh button.
2) Don't autoload options unless they are really needed on every page load. And if you do autoload the option then make sure it isn't too big 10KB should be plenty. Keep in mind if you create your option for the first time using the
update_option(); [codex] function it will be autoloaded by default. So you should instead intiate the option using
3) Don't use
wp_postmeta fields for numeric calculations. WordPress strongly encourages developers to work within their custom post type API, which is good, but that doesn't mean the Metadata API is equally good for all data types. For instance a post_type, "order", in which the record will have fields like 'subtotal','tax','discount','grand_total', which will likely be summed and counted and multiplied etc, go ahead and use a custom table with a foreign key on post_id.