This oven has a hidden bake element. This feature will cause it to heat up slower. If the oven eventually heats to the proper temperature and properly maintains it, then you may not have a problem.
Check to see if the broil element works properly. If the broil element is bad, then it will take longer to heat up. The broil element normally cycles on briefly during the bake cycle. The elements alternate so a problem with the broil element could cause the oven to heat slowly in the bake mode.
If the broil element works properly, I recommend that you
unplug the range
and check the bake element for damage. You can access the bake element by removing the thumb screws in the back of the bottom oven panel and the pulling the bottom panel out. If you find no damage to the bake element, then you could have a wiring problem or a bad electronic control board that is causing this problem.
I recommend that you have a service technician diagnose and repair a heating problem in your range. Here is a link for the Sears Service website:
Sears Home Services
If you need more help, reply with additional details and we will try to assist you further.