How many tables were available? How often were the available tables idle?
Here's a different format that might fix part of the problem:
The first phase of the tournament is the qualification phase. You run a bunch of 4-player single-elimination groups, race to 2. The winner of the group gets 2 points, the second place finisher gets 1 point. As soon as you are eliminated from a qualifier, you get back in line for another qualifier if you want to play some more. You stop the qualifiers at the end of the second day. All the tables are kept busy all the time.
The top 8 or 16 points earners come back on the third day to play a single-elimination tournament. That's three or four rounds in about 12 hours, so four or three hours per round, less meal time. You only need time control measures for that final day. You could play races to five, maybe.
In the qualification phase, the slow players are penalized by not being able to play as many qualifiers, so they won't have a shot at as many points. A slow player can only screw up two people in his group, and doesn't make a whole round wait on him.
There are a few details to worry about such as how to mix up the groups so you don't have to play the same person multiple times, but that's not a large problem.