Running Magento Enterprise 1.9, I noticed that with 20+ items in the cart the site was becoming extremely slow. Like, "10 seconds just to load a page" slow. As I increased this to 100 items the site became utterly unusable.
And this is not even on the cart or checkout page! It's on any page.
What can I do to mitigate this performance impact:
1) Globally, on all pages?
2) On the cart/checkout pages?
Too many observers observing and commenting on every line of product they observe in the cart. If you don't do downloadable products or muck around with RSS, these modules can be set to active=false in their control files to turn off the modules, put the observers out to pasture and speed up the cart function.
Their overhead is phenomenal... and causes the checkout to get exponentially slower as the number of lines increases.
app/etc/modules/Mage_Downloadable.xml set <active> line to
app/etc/modules/Mage_All.xml find <active> line for Mage_Rss and set to
The cause for this was sussed out by Rafael Kassner http://www.kassner.com.br/en/2011/01/07/magento-slow/
Standard disclaimer, YMMV, test on a dev server, etc. before unleashing on your live server.