The I2C fast mode defines transfer rates up to 400 kbit/s whilst the first I2C specification dated 1982 had a limit of 100 kbit/s. To accomplish 400 kbit/s the timing requirement were defined more strictly.
In May 2006 NXP, formerly Philips introduced Fast-mode Plus allowing up to 1 Mbit/s.
Additionaly, there is the high speed mode running the bus with with up to 3.4 Mbit/s.