to save resources I thought of switching on/off the physcis attribute of an entity.
Given that the velocioty of the object is below a certain margin I would decide to "mute" the object; when it would behit or touchend by some entity (via IMPACT) I would switch it on agaon.

But what is better in terms of performance:
To register / deregister an entity
or to use phent_enable instead?

you don't have to do this yourself. there already is a built-in sleep feature. -> ph_setautodisable()

thanks Ventilator