I think it is acceptable to wait for a search warrant to search someone's house, whatever that person was doing there, chances are that there will be traces of it even if the person tried to destroy evidence (blood residues, fingerprints, smells, etc).

But for a smartphone, I think it works a little bit differently. Any evidence that you might have on your smartphone can be quickly destroyed without leaving any traces. Even if a search warrant takes 5 minutes to be issued, by the time it arrives to the police it will be completely pointless, since you were able to press delete on all your photos during those 5 minutes.