Unfortunately I cannot help you much on the newsletter aspect as we handle all our email campaigns via CampaignerPro. As for categories and products those are mostly SQL bottlenecks mainly due to bad SQL joins (or missing indexes). Your best bet is to go into
top.inc.php and set the following:
PHP Code:
define('BENCH', true);
define('BENCH_BACKTRACE', true);
define('BENCH_DISPLAY_TYPE, 'A');
This will help you identify where a large number of bottlenecks are (anything in red for memory or time means something is wrong). The backtrace will let you know where exactly in the code it's at (these are available in the table's row for that entry so you'll need to use firebug inspection to see it)