Yup, a lot of force required to move them. Never going to move them by hand or even with pliers, you need to stick 'em in a vice and use something that's going to give you serious leverage before they will budge.
This part of the 2.5" piping installation is annoying simply because it needs to be done. ;-) One of the many things you don't really know about when you buy the piping - unless you did install research before buying.