Is there already some information on how big the performance impact is of having condensing activated?
I suppose having to do all these calculations needed to render condensed staves might slow things down a bit.
Are we advised to wait with activating this feature for the end of the creative process?
Yes, you would be well-advised to follow the steps described in the Version History: first write the music, then enable condensing. If you work in galley view and don’t have the condensed view open then there is no performance impact to speak of, but even editing non-condensed music in a layout with condensing enabled will be noticeably slower.