Preview in the app is not hardware accelerated or optimal. That’s just the way it is, take or leave it. Make a test export and play it player to verify for yourself that the output is fine.
If you want them smoother when playing in what I call the “preview window” you can try clearing the option “Realtime (frame dropping)” in settings. It should be smoother since no frames are dropped, but it will not play in real-time.
I did clips with and without having gone through Handbrake, and they are both smooth. So I guess Handbrake is not needed. Also, I found out its my PC that doesnt have the power to play the preview smoothly. Thanks for the reply.