What is the maximum allowable rise time for general inputs and clock inputs?
Some older data sheets (prior to Virtex-II) have a Tin value which indicates the maximum allowable signal transition time, but it is not published for newer devices.
The Tin is not specified for as it represents an endorsement of poor design techniques.
Generally, slow input transitions should be avoided for the following reasons:
In a slow design, combinatorial signals are allowed to be slow. Clocks should always be fast, as any ringing or ground bounce can lead to double-triggering.
Generally speaking maintaining montonicity in a signal transition is the most desireable solution and is easiest to achieve with faster edge rates.