Macbook Pro (Late 2010) slow after installing Mavericks
As Mathias711 pointed out in the comments, the cause of this was Spotlight automatically re-indexing after the after the OS upgrade. The process of indexing was greatly slowed down by OSX attempting to index a very slow, old external drive we had hooked up.

Once the indexing had finished (it actually took quite a while) the laptop eventually returned to its former speed. 

There's also a post by Computer World that backs up this conclusion:


  OS X will rebuild the Spotlight index after a major System upgrade. The process takes time and degrades performance while it takes place.
  
  Click on the Spotlight (looking glass) icon to the top right of the display to see if indexing is taking place. If indexing is in progress you should see a dot in the middle of the looking glass icon and/or be told the operation is taking place. Allow the process to complete and you should see immediate performance improvement.