The purpose of the attack on the Blight fleet wasn't to slow it down, it was to destroy targets that could have destroyed the Tines' world. The attack specifically targeted kinetic kill vehicles, those that were gradually accelerating themselves to large fractions of the speed of light. If the expedition to the Tines' world had become mired in another boundary surge in the Slowness, those kinetic kill vehicles would have destroyed the countermeasure before the OOBII could have reached it at sub-light speeds. Even without another unexpected Zone surge, the kinetic vehicles could have destroyed the planet shortly after OOBII's arrival anyway.
Remember that Old One didn't know what the countermeasure is. Old One was a Transcendent bottom-lugger, relatively insignificant compared to the origin of the Straumli Blight. It was not aware that the countermeasure was capable of hacking the Zone boundaries in the spectacular way that it did, only that there probably was a countermeasure the Blight was intent on eliminating. Pham's godshatter was not protecting the Tines or the main characters as such, it was merely doing everything in its power to ensure the safety of the countermeasure, whatever it was.