As you can imagine the biggest line expense in a consultancy P&L is people and therefore if revenue line falls that's the biggest lever they can pull to to maintain profitability or even survive.
In the US - after a justified re-sizing in early 2000s and some over-reacting in 2007 - Bain, BCG and McKinsey have become more careful in deciding on how soon and how strong they pull this lever. This because they realized that once talent is lost is hard to get it back and not having that talent available will stifle growth when there is an economic rebound.
However when billability falls under a certain treshold action is taken swiftly and low/average performers are outplaced more aggressively.
Hope this helps,