Kevin Chant has a few reasons why you might see slow clustered index rebuilds in your environment:
I better point out that online rebuilds in general tend to take longer. Mostly because behind the scene’s it’s making a rebuilt copy of your index and then it swaps around to the new index once it has completed.
However, there is another key point I should mention here.
Kevin also points out a sub-item for online rebuilds which could fit just as well in offline rebuilds: if there’s a long-running transaction which blocks SQL Server from taking the schema modification lock, you’ll be sitting there until those long-running transactions ahead of you finish.