star trek - Why would the Enterprise travel so slow to rescue a bridge officer?

Memory Alpha explains that:

By the time the Galaxy-class starship was being designed in the 2360s,
warp technology had progressed to the point where speeds of warp 9.6
could be sustained for up to twelve hours, although warp 9.2 was
considered the "red line." (TNG: "Encounter at Farpoint")

This quote indicates quite clearly that there are limits to how long warp drive can be sustained for.

The below image from the Star Trek TNG Technical Manual confirms that an increase in warp factor requires an increased amount of energy:

(Source)

And, as explained in this question, antimatter is not produced by starships themselves, so Captains and engineers need to be wary of antimatter supplies as well. This is addressed quite well in this question.

As to why Picard decided to travel at a slower speed and leave an officer at risk for slightly longer, Picard needs to weigh up the safety of his whole ship compared to the safety of one officer. As Spock so famously put it: "The needs of the many outweigh the needs of the few or the one." Why risk the entire ship for just a couple of hours delay when there was no evidence to suggest that such a delay would jeopoardise Data's safety?