One person's heavy traffic is another person's light traffic. If you're from a rural area where the biggest travel headache is having to stop while the school bus picks up kids in the morning, traffic in Orlando will likely seem a bit daunting. Even on a Sunday morning, that person might be surprised at the amount of traffic. (There will be a decent amount of traffic at that hour because many hotel/theme park workers will be scheduled to start shifts at 8am.)
On the other hand, if you're like me, driving in Orlando is a breeze. I grew up in Los Angeles and currently live in South Florida. Both of those locations have lots of traffic. Back when I lived in Orlando, I lived near Universal and worked at Disney World. Even at prime rush hour, it was extremely rare to get stuck in "dead stop on the freeway" traffic.
In the end, I wouldn't stress over traffic. It is what it is. Assuming you want to get to your Disney Resort and try to get to a theme park first thing in the morning, leaving at 7am on a Sunday should easily accomplish that goal.