If you read the making of book, you'll see that they had ALOT of new programmers on this film, people that had never worked on a Star Wars film before. There was a good amount of corrections that had to be done by GL because things were being done graphically that didn't fit the 'laws' of the Star Wars universe. Also, alot of the CGI stuff towards the end was cut or reduced, probably this battle included, because of a strapping of cash. They wasted alot of money CGI'ng scenes that Lucas eventually cut because he wanted the movie to be more about Anakin. Thus, when it came time to do some of the additional effects work that was left for last, they were trying to get it as good as they could in just a few mockups because the dollar total was increasing each day.