FPS is the rate at which frames appear on display. Higher rates mean GIF/Video becomes more smooth, while the lower rates, the less smooth it becomes.
On ImgPlay, when you make FPS higher, the video length becomes shorter, this is because ImgPlay has a limitation on the maximum frames to edit on ImgPlay to protect your device from using too much RAM memory occurring crash. Below is the maximum frames for each device.
Max 100 Frames : 4, 4S, 5, 5S, 6, 6 Plus
Max 150 Frames : SE, 6S, 6S Plus, 7, 8, XS, XS Max, iPad
Max 200 Frames : 7 Plus, 8 Plus, X, iPad Pro, 11, 11 Pro
The number of the maximum frames does not mean the length of the GIF/Video. This is the number of frames you can use when you create GIF/Video with ImgPlay. With the same amount of the frames, you can make a long GIF/Video but less smooth or a short GIF/Video but for more smooth.