Originally Posted by Retro Grouch

Why that's easy. As rear cassettes crammed more cogs into the same amount of space more precision is required for a rear derailleur to index properly.

If a cable is kinked or a housing messed up I obviously fix those things first. Generally if someone brings a bike to me that has them frustrated they have already checked or replaced those items. Checking the hanger alignment only takes a few minutes and, even if it checks out OK, I've ruled out a common cause for poor shifting.