4.3 - selecting item in hierarchy causing long freeze/lag in inspector

We've updated from unity 4.2 to 4.3 yesterday and tonight we've noticed a very serious lag/freeze issue that seems to be related to the inspector - whenever we select something in the scene hierarchy, Unity becomes unresponsive for 30 seconds to 1 min before eventually showing the inspector values for that object. Selecting things (folders etc.) in the project's assets is as quick as it always was - the freeze/lag seems specifically to do with the inspector locking up.

Interestingly, we've just noticed the same problem with our Mac Mini, so there's a possibility that it's to do with our project, but everything was working fine (in 4.3 on the PC) up until last night (Unity was restarted since then). Would anoyone have any ideas that could fix this? As always, it's very urgent :S

 

I also have problems with this, just clicking on things not even in the hierarchy view but in the project view, especially if I click on a scene file, Unity acts as if I just tried to load the scene or object and it lags horribly. Locks me out from 10-30 seconds. Also, when playing scene, the playback is horribly laggy unless I first click off any objects in the editor. This has been confirmed by others in my team. I have updated my graphics drivers to the latest and I have a really decent system. 12 gig Ram, GTX 680 with 6gig of graphics memory.

 

Update, found out the lag I was having was having the Atlas Maker widget for NGUI open.