Super fine grain developers (like Microcol-X) always cut speed. Might try that. Don't dilute, use full strength. If diluted, they don't cut as much speed.(posted 8636 days ago)Before doing this, make sure the camera setting was actually used. (In a home studio, you might use flash or hand held meter.)If it was flash, but not "through the lens flash metering", the setting on the flash itself would have prevailed (in auto mode).
The Massive Developing Chart shows Perceptol for 9 mintues for 25 speed. This might work.