The Constraints Guide states that it is possible to set I/O configuration constraints, such as fast and slow, in the Constraints Editor Port tab. However, I cannot find the entry in the Port tab of the Constraints Editor.
The I/O configuration constraints entry is removed from the Constraints Editor in ISE 7.1i. Alternatively, you can use PACE to do perform the same function. The content in the Constraints Guide and in the Constraints Editor Help is incorrect and will be updated in a future version.