Interesting - that 25% seems to come from managers estimating the impact, not from developers.

  Increase in time taken to code the feature because 15–20% 25–35% 15% 25–20%
  of TDD (%) [Management estimates]

And this seems to imply that any increase in time was offset by reduced maintenance costs due to improved quality.

  Another interesting observation from the outcome measures in Table 3 is the increase in
  time to develop the features attributed to the usage of the TDD practice, as subjectively
  estimated by management. The increase in development time ranges from 15% to 35%.
  From an efficacy perspective this increase in development time is offset by the by the
  reduced maintenance costs due to the improvement in quality (Erdogmus and Williams
  2003), an observation that was backed up the product teams at Microsoft and IBM.