This will need to be investigated further; I tried some scenarios myself yesterday but was not able to get the device into such a state that it would take as long as you described to open the screen lock. However it seems that the wakeup from sleep state requires a lot of power from the cpu, which could cause QSpot to become unresponsive (I have noticed that it could take some time for QSpot to "recover" from sleep state, but once wake up has completed it works just fine). I almost daily listen to QSpot while sitting in the bus for close to an hour (over 3g, low / normal bitrate; with a buffer length of 300secs), and I have not noticed that the device has been unresponsive once I try to open the screen lock.

Have you used some custom buffer length (other than the default value of 30secs)?