The two methods for isActive in the helper Data.php are used excessively...

... in the event core_block_abstract_to_html_before, so caching them improves performance a lot!
On my site, the homepage needed 2,6 seconds to load. After this fix, it was reduced to 1 second. It works, because Mage::helper() is a singleton and thus the variables can be cached over events, so all the computation is only done once instead of for each single event.