The folding rate of single domain proteins following two state kinetics can differ up to millions of times in magnitude. Plaxco et al. in their paper "Topology, Stability, Sequence, and Length: Defining the Determinants of Two-State Protein Folding Kinetics" summarized their experiments and findings on which of the factors cause such a spread in folding speeds. They found that folding process is not very sensitive to the finest details of the sequence, but rather it depends on the large-scale topological properties of the sequence.

Reference: KW Plaxco, KT Simons, I Ruczinski, and D Baker. (2000). Topology, Stability, Sequence, and Length: Defining the Determinants of Two-State Protein Folding Kinetics. Biochemistry. Sept 19; 39(37):11177-83.

The paper is available at http://www.cs.ubc.ca/labs/beta/Courses/BioinfoReadingGroup/Winter2003/plaxco.pdf