What are the most important factors in a layout for typing speed?

I think they would be:

• Same finger bigrams (of course)
• Low same finger skipgrams (SFS, DSFB) - Like same finger bigrams but separated by letters in between, e.g. m_y in may on QWERTY.
• Low fingerspeed - To reduce how much your fingers need to move to maintain a high WPM

Are there any other factors that are important in a layout for typing speed?

What layouts are the best for typing fast?