If the answer to Reaper's question is YES, the magazine was out... then we'll need a better description of the force it required. If you held onto the slide too hard while engaging the takedown lever you didn't let the slide detach itself naturally via the spring. Your results may vary, but when I field strip my 92FS, I keep a light grip on the slide and let the slide TRY and fly off the frame.
How did putting the slide back on feel?
Also, since it is brand new, the slide/frame might need some range time to get used to each other. It won't take long at all for that.