Maximum Render Quality CS5.5

My project is about 1hr 15 mins long, covering 8 sequences.

3 days ago, I encoded using the MPG2-DVD preset and burnt a trial disk with an Encore project.

After reviewing the disk, I made a few trivial changes to the PP project and encoded again.

Exactly the same encode settings except I checked Maximum Render Quality.

This time, it took FIVE TIMES longer to encode, and I cannot see any difference in disk quality.

What does checking Max Render Quality do? 

Would you expect it to take 5x?

And would you expect a better quality disk?

Thanks