A while back I did a mod for the hypershade window to add a button for toggling the updating of the thumbnails. Turning off updates can improve performance when you are modifying complex shader networks. I've just added a version for maya 2012. The hyperShadePanel.rar on the downloads page now contains 2 new hyperShadePanel.mel versions - for 2011 & 2012 mel script (put the one you need in your local scripts folder) and 2 new bitmaps in the png format (put both of these in your local icons folder).
update: added 2011 version to the rar file, as well as fixing a small autodesk created bug in the 2012 version where the width of the create tab always took up half the window when first opened (bug fix thanks to Ingo Clemens). If you downloaded in the last last 24 hours you may want to get this update.
update 2: Added improved thumbnailUpdatesGreyed.png icon for the button disabled state - created by Mike Smith-Kennard (thanks MSK)
update: added 2013 version