[RPG] what is considered a “fast pace” for flying creatures

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:

A fast pace makes characters less perceptive, while a slow pace makes it possible to sneak around and to search an area more carefully (see the "Activity While Traveling" section later in this chapter for more information).

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:

... a slow pace makes it possible to to search an area more carefully (see the "Activity While Traveling" section ...

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):

Characters can gather food and water as the party travels at a normal or slow pace.

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.

Ability Checks

The DM calls for an ability check when a character or monster attempts an action that has a chance of failure. When the outcome is uncertain, the dice determine the results. (PHB p.174, "Ability Checks," emphasis mine.)

When deciding whether to use a roll, ask yourself two questions:

• Is a task so easy and so free of conflict and stress that there should be no chance of failure?
• Is a task so inappropriate or impossible--such as hitting the moon with an arrow--that it can't work? (DMG p.237, "Using Ability Scores," emphases mine.)

So the rules say that the GM should only allow your perception checks in situations where it would even be possible to perceive something. On top of this they might be disadvantaged, or penalized, or have DC-adjustments. In other words, it's up to your GM to decide "\$Y\$ is the altitude above which I do't even have you make checks, \$Z\$ is the altitude above which those checks are disadvantaged" or however they'd like to handle it.

Do they apply in your hypotheticals? Impossible to say without considering, as DaleM does in his answer, the sizes of objects and your flying height and speed. Really, this is one of those situations where you and you GM might spend an afternoon at a coffee shop trolling Wikipedia for information on visual acuity.

I do know one thing, though: If you're putting a lot more mechanical granularity into your character's vision than your table-mates are into their characters' vision, be careful. In that situation you're actually playing with a different quality of playing-piece than the rest of your table-mates, which can lead to tension down the road.