Why does the Bitcoin core client use so much CPU time while synching? I have always noticed the huge increase in cpu usage when I opened my client for the first time in a few days. I was wonder what this is from? I would imagine verifying all of the signatures of the blocks? 

Essentially doing the same calculation the miners seek to do and just checking their answers?
I would imagine verifying all of the signatures of the blocks? 


You are verifying every single script, every single signature for continuity. You are making sure nobody is spending coins they're not allowed to. You're making sure all the rules are followed. 


  Essentially doing the same calculation the miners seek to do and just checking their answers?


Yes, in addition to the above. You are verifying every block has a valid Proof Of Work. This takes two SHA256 operations though, versus the billions of billions a miner needs to do.