Whenever I apply a motion tween (especially when the object moves fast), the result is always so jerky that it almost makes one's eyes bleed.
For example - try following the motion of the box:
Obviously, I can fix it by increasing the frame rate - like three times, or so. But I was hoping to avoid it, because file size matters a lot for my purposes. I was hoping that there might be some way to smoothen the animation and make the motion more eye-friendly.
Thanks in advance.
Try using a classic tween instead of a motion tween.
It's nothing to do with the tween and everything to do with frame rate and how you build your movement.
You can't have smooth movement at low frame rate and large amplitudes. There is just not enough visual information for this.
If you need that kind of constant steady movement back and forth at low frame rate you can try to apply some motion blur or "dry brush" effect.
If you only have to go from point A to point B I would suggest an antic and a settle with easing and small amplitudes, while skipping the middle part.
If you messed up a tween the best thing to do is start over. Just delete the tween and redo it. I know it stinks but it is for the best and will be less of a headache.
That motion looks pretty smooth to me for the frame rate you are using - looks like 24fps? And yes, I get it. When publishing to GIF format, frame rate is critical. But you need to find that balance between low frame rate and smoother motion. You will have to sacrifice something somewhere. But there is no "smooth motion" feature you can select to help here.
The one thing I've found impossible to animate smoothly is rotating an object. If you're trying to rotate an Object with a Tween, try something different. (For example if you are giving some palm leaves a gentle sway in the wind, move them up and down, don't have them Rotate.) Distinct from that, try making your Object smaller and/or less complicated - anything to make it easier for Animate to rasterize.