On PHB p. 50, the Totem Warrior Barbarian's Wolf totem for the 6th-level Aspect of the Beast feature grants the following benefit:
You gain the hunting sensibilities of a wolf. You can track other creatures while traveling at a fast pace, and you can move stealthily while traveling at a normal pace (see chapter 8 for rules on travel pace).
On PHB p. 182, the Travel Pace chart says that you can normally only move stealthily at a slow pace, but there is no mention of a restriction on travel pace for tracking. The stealth benefit for the Wolf aspect is useful at least. A fast pace causes a -5 to passive Wisdom (Perception) scores only. The Track activity under "Other Activities" on PHB p. 183 states:
A character can follow the tracks of another creature, making a Wisdom (Survival) check when the DM calls for it.
Again, there is no mention of travel pace being a factor. A check of the DMG section on tracking, DMG p. 244, lists several modifications for checks to track, none of which mention travel pace. Even the "Chases" section (DMG p. 253-255) doesn't mention anything where this part of the Wolf aspect would come into play.
Does the tracking at fast speed portion of the Wolf Aspect of the Beast adjust a game mechanic?
Best Answer
Tracking is mentioned only briefly, in/around the pages you mentioned, in both the PHB and the DMG. And as you pointed out, in the PHB and in the DMG it doesn't specify tracking as having any maximum speed.
On PHB page 181, under Travel Pace:
This sentence, depending on how you read it, could mean that any "Activity While Traveling" (which is the section Tracking falls under) can only be taken while moving at a slow pace:
However, several activities specify how they operate under different speeds, meaning this interpretation doesn't hold up.
Additionally, Foraging, another Activity While Traveling that points to full rules in the DMG, has the following statement (DMG page 111):
It makes sense that Tracking would have a similar statement, but it does not, so for the time being there's no RAW restriction on tracking at a fast pace.
You can, however, interpret it as being RAI due to Foraging or due to the negative implication of the Barbarian ability.