Why does it in some cases take so long to receive a track response?

Check on Me uses the Google Push service to advise the tracked phone of the track request. Similarly, once the location is determined Check on Me uses the Google Push Service to advise you of the outcome.
Google does not guarantee delivery times when using its service. In most cases it is reliable and fast (approx 30 seconds) but you should never rely on that.
Another reason for a delay may be that the tracked member has temporary disabled tracking. Your tracking request is only executed after the member re-enables tracking again.