It's not just speed. This also improves the usability of vim by turning insert mode into a pseudo-mode, which is a mode that requires constant interaction (depressing the pedal) to be maintained.
Pseudo-modes are generally much less error prone than normal modes, the canonical example being the shift vs. caps lock keys.
Do you have any literature on this being called a pseudo-mode and the tradeoffs between it and "real" modes? I am currently pretty interested in these kinds of topics.
I'm sorry, I don't know of any online resources (and after googling, it seems the more common term is "quasimode") but I can heartily recommend the book The Design of Everyday Things by Donald Norman. It's a seminal book on designing interactions between humans and objects, and was required reading in my human-computer interaction class in college.
Pseudo-modes are generally much less error prone than normal modes, the canonical example being the shift vs. caps lock keys.