The time cost is the whole point. It slows the players down ...sort of like slow travel. Pointless irritation. If I want to explore, I will, and I do, more often than not. If I want to get somewhere *now*, it's pointlessly irritating to make me go through the intervening terrain at a slow clip and/or via a winding path.
Time is what gives a sense of scale. As strange as it sounds, we measure distance by time. To walk a mile is longer than to drive a mile. A mile isn't a mile, a mile is x / v = t. Of course necessary effort, boredom, and incentive matter. We can easily prove this. Run your character from Stormwind to Ironforge by the quickest route that doesn't use the tram or portals. Pretty far, isn't it? Now take the tram. Seems a lot closer. You did actually run a shorter distance, thanks to not going around several mountains, but even if those weren't there, it feels shorter, doesn't it? Now take a portal. Notice how x is unchanged, but v gets faster and faster, reducing t to almost nothing.
Let's move away from that for a moment and think about time the way we normally do, as a blank wall which someone is throwing paint at*. Or as money. Whatever. Something limited.
When is my time most valuable? Trick question, the relevant person isn't me, but those around me. Put me in a group and the time cost is multiplied. I'm not waiting alone, a whole group is waiting. Logically we would want to minimize the time cost when people are in groups compared to when they are alone. In this case we would want to instantly teleport people to groups while using slower mounts and flight paths when they are alone. Finally, since groups often get us killed, make death faster. We already have this with teleports to instances. There are also several group-based forms of rapid transportation: portals, warlock summons, and summoning stones, all of which put people in the same place or bring them together, very quickly.
To take it further, we could identify times when time is particularly valuable, even when not in a group. For example, when doing daily quests we may be sort on time, hence picking short-term tasks which take small amounts of time. Due to the low supply, time is more valuable. Furthermore, any added time from travel is a higher proportion of the total time, further inflated the relative cost of travel time. So we have portals to, or very near, common quest hubs. Finally we may need to log off suddenly or at precise times. In this case, quickly getting to a safe area, preferably one with rested xp, is an urgent matter. And so we have hearthstones.
It appears that WoW has a sensible travel system in regard to minimizing the use of time when it is expensive. This manifests in giving several forms of teleportation when in groups, doing dailies, and logging off. Contrast this with foot or mounted travel when questing, a normally solo and unrushed activity. Logic says that WoW has a good travel system. I don't much like it. So logically, I am illogical.
t = x / v
t = x when v = 1
But what is 1?
* back in college I attempted to use this visual to explain my perception of work, chores, and relaxation. The general idea was that a blank wall is nice (day off). A painted wall is nice (work). But a wall with random splatters of paint is not nice (chores/modern art). Everyone thought I was crazy. Not a good way to start my first psychology course.