My blackberry takes 15-20 minutes to power up onces switched off. Anyone had this problem? Any ideas?

How are you switching it off? A full boot up from a no/dead battery is quite long, but the device is not designed to start from this state frequently.
The soft power off (holding down the power button on the top) puts it into more of a standby and shouldnt take more than a couple seconds.

Hi,

go to messages, options- view folders - phone call logs, and clear all phone log and missed call log.

This will make your device faster, even boot will be faster.

Wow and I was getting impatient with 2 minutes on mine!!!