Turns out indexing for sharepoint search was set to reindex every 5 minutes, causing 100% cpu on sqlservr.exe. This in turn slowed down the sharepoint portal significantly.
To set reindexing of sharepoint search to a more sensible time (out of hours) open up sharepoint central administration on your sharepoint server. In “Farm Topology” click on the server name that is running “Windows SharePoint Services Search”. Now click on “Windows SharePoint Services Search” in the list that pops up. Now go to the “Indexing Schedule” section and set the indexing schedule to something sensible, like daily between 12am and 12:15am (or whenever the server is not being heavily used).