And that, in addition to "What Which Survives", is the most extreme high outlier in the TOS material. But the bright side here is that TNG, DS9, VOY or ENT material in turn features extremely few low outliers to contradict this.
Basically, we only have annoyingly low definitions of warp factors 3 ("The Most Toys") and 4.4 ("Broken Bow"), both of which are also in direct contradiction with the rest of the material from the very same episodes (warp 3 in "The Most Toys" allowed a choice of multiple star systems within a day, just as it generally should, and warp 4.4 in "Broken Bow" took the ship to a side trip of 15 lightyears in no time flat).
Generally, TNG is not really slower than TOS. It's merely accompanied by a more explicit source for a "backstage interpretation" of warp speeds - but that doesn't make the backstage source any more believable or consistent with what we actually see on screen. The practical difference lies in how very long voyages are treated. A dozen or a hundred lightyears can be covered very fast in both TOS and
the rest; ten thousand will probably
involve lower average speeds in both cases, but not assuredly so.
That is, we don't know of TOS examples of journeys of ten thousand lightyears, but the one that took Kirk to the Galactic Barrier for the pilot episode may have been one of those. It happened offscreen, after all, and may have involved months or even years of outward travel. Certainly the stardates for the subsequent episodes suggest it took Kirk a long time to return to civilization afterwards...
The "in-between" journeys of about a thousand lightyears are where the true difference lies. "That Which Survives" shows a high average speed for such a journey, but "Q Who?" shows a much
lower average speed for a journey merely a couple of times longer. But that is solely the "fault" of the TOS outlier here, as most evidence in Trek suggests that average speeds drop drastically after the journey has taken a couple of hundred lightyears, or lasted for a couple of days.