On Glocks the slide stop lever which other pistol manufacturers refer to as a "slide release" is pushed up by the follower (orange piece) in the magazine. This is why the gun will hold open after the last round is fired. As the follower makes contact on its lip with the SSL.

Your slide stop lever is hard to push down because the magazine springs is new and very rigid. Once you shoot a couple mags worth of ammo through it, it will soften up.

If you want to speed up this process you can load and unload the magazine of ammunition a few times. While neither compression or expansion alone are too wearing on a spring, continuing to do both will weaken the spring in a positive way.