Have you tried lubricating the hinges? Though, take care to use a product which wont damage; the hatch's gasket
, it's deck
gasket/bedding compound, nor the lens & or it's adhesive/bedding.
Also, might there be a possibility that the shape of your deck
has shifted slightly, thus changing the shape of the hatch or it's deck frame, ever so slightly. And thus causing the binding/heavy opening pressure.
It'd be worth checking this with a framing square, as well as measuring the hatch across it's diagonals, in order to see.
Have you re-tuned your rig recently? Which could also cause a small change in the shape of the deck. Ergo the stiffness.
If none of those options yields much, & it's still bugging you, it might be time to disassemble the hatch. And the step after that is to pull it out of the boat completely, & do a re-install.
Also, I hate to mention it, but water
could have gotten into the deck core
over the Winter, frozen, and again, resulted in a small shape change in the deck.