Slow motion is just 120fps video, which will look normal speed on a PC or even through a 3rd party video player.
You have to open the Samsung Gallery app and find the slow mo video with the slow mo icon. This will open it in the slow mo player.
In the slow mo player, you can select the slow mo points and the speed, 1/2x to 1/8x, but natively 1/4x (1/8x is a bit stuttery as it's only 15fps). Then choose the Export option to save the file with the slow motion effects. The exported file can be viewed anywhere.
This is different from the S5, which saved the recorded file in slowmo.