I find that clicking "Play" directly in Photoshop isn't representative. To have a better look at the result, you need to click "Save for Web", and then watch your animation there.
changing the speed by decreasing the amount of frames in the Frame Timeline can result in choppy-like animation due to it being frame-by-frame based, whereas you can change the duration with ease in the Video Timeline and get smooth animation.
hence why I suggested to use the Video Timeline instead.