If you are taking the bus, or any other mode of transport that drops you off at the front of the park, allow enough time to walk to World Showcase restaurants. It's a little bit of a walk from the bus stop to the front gates, and then another 15-30 minutes walking, depending on where you are dining, and how fast you walk. Add that to the 15-30 minutes to wait for the bus and to ride it, and you can see you'll need to leave WL at least an hour before your ADR time.
If you happen to be at another park instead of at WL on the day of your dining reservation, it can be quicker to take transportation to the BC and walk in through International Gateway. But this is only faster if you are heading to Epcot from another park and not from your resort.