Help Very slow start with Virgin Mobile

[pibs]

Since changing to virgin Mobile, my HTC Desire S phone takes about 4 minutes to start and then everything works in slow motion lets say, for the next few minutes. I noticed this in the shop when the assistant tried a SIM card to see if my phone was open for Virgin or not and he commented that it was taking ages to start. This didn't happen with other SIM card with another network but did take a while for a restart. I haven't done a factory reset as didn't want to lose info stored on the phone and apps etc but may have to unless a remedy cn be identified by the experts on here. Many thanks.