State and federal governments arguing "it's really hard and expensive so you shouldn't make us do it" is the default response to pretty much any FOIA request. This instance isn't special.
How large were the documents you requested that needed review? Was it the first time they were being reviewed?
Edit: also, I haven't looked further into this but can almost guarantee you that the FBI was trying to work with the requester to narrow down what they're requesting to something that is slightly more focused than hundreds of thousands of pages that all need review first
