I am just started with Houdini and I am following this tutorial: https://www.youtube.com/watch?v=9Agq4ZOm2rc&t=294s [www.youtube.com]
Because my spheres are moving extremely fast with the same settings. If $FF is hardware dependent, what expression can I use to make it basing on velocity/time, like say move 1 unit up every 1 second? (like Time.deltaTime in Unity)
Many thanks!

Turn on realtime playback mode. The little clock icon on the playbar to limit it to play no faster than the scene framerate.

$FF is in frames, Houdini is not a realtime application so real time has no meaning in it. The only thing that matters is the scene time.