Is your grill single or double walled? Double Walled grills (like the Timberline Series) hold the heat pretty well and don't need an insulated cover for cold weather cooks. I see you are from Canada and if you use your grill all year long you may want to invest in a thermal blanket if your cooker is single walled.
The next thing you should do is determine your actual grill temp vs the grill temp shown on your controller. By checking actual ambient temperature of the grill with a FireBoard II Ambient Probe, I discovered my Timberline actually runs 9 degrees hotter than what is posted on my controller. So its quite possible your grill could be cooking at a lower temp than what is shown on your controller and that could be another reason why your cook is taking so long.