Possible related issue...
Observers for Downloadable Products and RSS cause a noticeable slow down as each gets triggered for each line item on the Sales Order. At about 20 items, the wait is excruciating. Here’s the module config.xml snippet that fires the observer for Mage_Downloadable.
Since I use neither of these modules, I decided to fire a Sparrow up their tailpipes and end the agony. In 184.108.40.206, you can do this by finding the <active> tags and changing the contents to false in:
and in the Mage_Rss section in:
Instead of merely disabling the output and leaving the observers alive as resource sucking undead, this shuts the modules down completely. This can be done with any Magento module, but not with impunity. Be sure to check the <depends> tags for each module to make sure any of the ones you disable is not needed.