DGW explained it...twice.
When the magazine is empty, the spring is pushing up on the stop. That's why it's hard when the magazine is empty and easy when it's loaded or when the magazine isn't locked into the gun.
And the notion that it's not a slide release is ludicrous. Why does it protrude from the side of the frame AND have serrations or checkering if it's not meant to be used to release the slide?
Actually...not counting takedown...the slidestop has five functions.
It's a slidestop that keeps the slide from flying off the frame when it runs forward.
It's a slide lock that holds the slide open when the magazine is empty.
It's a slide release used to chamber a round after a fresh magazine is inserted.
It's a camming surface that lifts the barrel into the slide
It's an anchor for the link to pull the barrel back out of the slide when the slide recoils.