You should be able to change the interval in the source and re-run. The initial poll interval was chosen to be kind to the Telldus Live servers. Their API documentation also mentions a polling interval limit somewhere.
I also have a door sensor and I think a polling based approach is inadequate for this scenario. The Tellstick Net device can be set to broadcast all events on the local network, in addition to communicating with the Telldus Live servers. This way it is possible for HA to get instant notification of sensor updates etc from the Tellstick Net device, instead of polling the Telldus Live servers. A first attempt to implement support for this is available at https://github.com/molobrakos/tellsticknet - please feel free to contribute!