Make Magento add to cart and checkout faster

Only replies with a reference to this project are taken into account. What is the problem: I have a dedicated server with a multisite (10 stores) webshop in Magento. The site itself is fast now. Although a lot of modules are used and we have 5000 visitors per day the pages load quick. But the add to cart and onestepcheckout and placing an order functions are really slow. Some things seem to speed this up, but I don't know for sure these really solve the underlying problem: - emptying cache (we also have FPC installed) - emptying sessions (these are saved to files by the way) - restarting mysql Every day we notice that some hours the shop works fine, but at some point the database or something runs too heavy loads, and at some points the normal homepage loads in like 2 seconds or less and the add to cart / onestepcheckout and place an order process loads in 60 seconds. Then a process is rebooted or something like restarting mysql is done and the site runs smoothly again. Hosting says we have a lot Joins performed without indexes: 32547, so maybe this is a clue. Also: is it possible to cache the external modules locally, like zopim? Maybe this makes it faster also the host says. If you know how to solve this, we need some extra hands in future for these optimization and server issues.... Please take a look at the screenshot from pingdom. You see the loading times fluctuate like hell at the [login to view URL] url Niels By the way: I only pay the milestone after the site works smoothly for 7 days in a row.... it's