this is indeed kind of a solution here, as it involves editing core xml files (it's my understanding there is no way to override those files by a custom module: someone please correct me if I'm wrong), but here it is.
See edit.
Acording to these guys (I haven't try it myself) there are 3 observers that add a lot of execution time in order process. Their solution is to remove/comment those lines:
In app/code/core/Mage/Downloadable/etc/config.xml:
<sales_order_item_save_after>
<observers>
<downloadable_observer>
<class>downloadable/observer</class>
<method>saveDownloadableOrderItem</method>
</downloadable_observer>
</observers>
</sales_order_item_save_after>
And in app/code/core/Mage/Rss/etc/config.xml:
<sales_order_item_save_after>
<observers>
<notifystock>
<class>rss/observer</class>
<method>salesOrderItemSaveAfterNotifyStock</method>
</notifystock>
</observers>
</sales_order_item_save_after>
<sales_order_item_save_after>
<observers>
<ordernew>
<class>rss/observer</class>
<method>salesOrderItemSaveAfterOrderNew</method>
</ordernew>
</observers>
</sales_order_item_save_after>
Maybe you can check if you can go without them and give a try. Remember that at next upgrade those changes will probably be lost.
Hope That Helps
Edit: At inchoo they found a solution to disabled events without editing core files, it won't be difficult to apply it in this case.