After you overlay Video, say in a Title, or PiP (Picture in Picture), or add/adjust an Effect, you should get a red line about the Clip(s) affected. To get the smoothest playback, one should Render at least that/those Clip(s), by hitting the Enter/Return key. Note: one can limit the area of the Timeline, with the WAB (Work Area Bar).
Hope that helps,
Thank you Hunt, your suggestion is correct.
I got a little bit confused by the "play" button, that didn't switch its icon to "pause". Moreover the animation looked smooth when I dragged the cursors forward and backward... It really seems that the system didn't start the playback, rather than just "needing a renderer". However now I know it!
Rendering is often mis-understood, as with some footage/Projects, and some editing operations, it is not really necessary for adequately smooth playback. I often have SD Projects, where I will never bother to Render anything, as it is smooth enough, without Rendering. However, and especially when doing Keyframed Effect animation, I might Render a section of my Timeline many dozen times, to test with the best playback. With HD material/Projects, especially if one does not have a "killer" computer, Rendering might be more necessary, and more often. This ARTICLE goes into more detail.
Good luck, and happy editing,