I am experiencing extreme lag when resizing (zooming in) the cycles node editor. I don't experience lag anywhere else in the blender UI. My specs: quad-core, 2.40 GHz 64-bit. No GPU. Does anyone have any solutions for this problem?
I think I figured this one out!
It's the rendering of the default interface FONT that causes the lag. It seems it has to do with the file size of the .ttf used by defualt in Blender (~5MB).
So in User Preferences->System->Interface Font, try switching to a .ttf font with a small file size. I used Lucida Sans at 53kb.
I'm not certain as to why the font's file size matters, but IT WORKS.
After reading ItsTheFont's answer, I played around with the settings and found that the Window Draw Method makes a big difference as well. It can be found at User Preferences > System > Window Draw Method. Setting it to Overlap was best for me.
My Interface Font setting is now set to OpenSans Regular.
A image showing where to find the Window Draw Method and Interface Font settings.