Lunch at any of the Village restaurants is never packed, as most people prefer to stay in the parks at lunchtimes. If staying in the parks over lunchtime, early lunch (noon) or late (after 1.30) is quieter in almost all the restaurants/cafes
Just before park closing time for the Village restaurants is also good. Try to avoid eating at the Village within the first hour of so after park closing time, as thats when all the Village restaurants are at their busiest. It can also get busy in the hotel restaurants around this time too.
Having said all that, you can book most restaurants at the onsite hotels, and some restaurants in the Village, and if you prebook your table, you'll be fine. I personally haven't had particularly slow service at any table-service restaurant at DLRP, regardless of how full the restaurant has been.
Elaine