At lower temperature, transistors become faster, so edge rates would become faster.
Generally, we would expect the following parameters getting affected as temperature decreases beyond the specified operating limit:
1) Decreased frequency operating range (for reliable locking)
2) Faster edge rates
3) Higher jitter
4) Duty Cycle
Operating a device outside of its design range is very risky. While it is possible that the only effects of operating at a reduced temperature will be shifts to some parameters such as duty cycle and edge rates, it is also possible that the device performance may degrade more seriously. It may also fail to operate reliably. For example, the PLL may not lock or may lose lock, or the spread profile in case of spread aware buffers could become much distorted.
