Well, it makes a lot of sense that long trips would have to be conducted at a lower average speed than short ones. Ships "tire" just like runners. Covering 100 m in ten seconds doesn't mean you could cover 100 km in ten thousand seconds or under three hours.
Four days to Vulcan in ST:TMP is a bit slow in terms of even the average TOS or TNG travel times, ignoring the outliers. Possibly Scotty was telling Spock how sorry he was that the ship was so tasked and damaged that it would take four full days to get the Vulcan back "home"?