In an I2C communication the master device determines the clock speed. Unlike RS232 the I2C bus provides an explicit clock signal which relieves master and slave from synchronizing exactly to a predefined baud rate.
However, there are situations where an I2C slave is not able to co-operate with the clock speed given by the master and needs to slow down a little. This is done by a mechanism referred to as clock stretching.
An I2C slave is allowed to hold down the clock if it needs to reduce the bus speed. The master on the other hand is required to read back the clock signal after releasing it to high state and wait until the line has actually gone high.
Clockstretching sounds a bit odd but is common practice. However the total bandwith of the shared bus might be significantly decreased. So, especially for I2C buses shared by multiple devices it is important to estimate the impacts of clock-stretching. So do not make the slowest I2C device dominate your bus performance.
Clock Stretching an High Speed Mode
Clock stretching in High-Speed-Mode is only allowed after the ACK bit (and before the 1st bit of the next byte). Stretching between bits 2-9 is illegal because the edges of these bits are boosted with an additional current source. See I2C specification Rev. 03 chapter 5.3.1 for further details.