Robert Jordan (pen name) is a perfect example - he took 6 years to write the first two books of his "Wheel of Time" series, and cranked the rest out like pamphlets.
I'm not sure where you got that idea:
1. The Eye of the World (15 January 1990)
2. The Great Hunt (15 November 1990)
3. The Dragon Reborn (15 October 1991)
4. The Shadow Rising (15 September 1992)
5. The Fires of Heaven (15 October 1993)
6. Lord of Chaos (15 October 1994) Locus Award nominee, 1995
7. A Crown of Swords (15 May 1996)
8. The Path of Daggers (20 October 1998)
9. Winter's Heart (9 November 2000)
10. Crossroads of Twilight (7 January 2003)
11. Knife of Dreams (11 October 2005)
The novels grew worse and worse by the year...
That's also not true. They got better while he was writing one a year, until book #6, and then they started getting progressively worse, as he took longer and longer between books, then they started getting better again starting with book #9, though, until book #12 (finished by Sanderson) they hadn't reclaimed the level of the first several books.
In Robert Jordan's case, what damaged his writing was not speed, but the fact that he put too many irons in the fire and was trying to tell too big of a story.