I'm working on an alt character. The alt has just entered HW at 53, and has hunts unlocked, including the first tier of the HW hunts. However, despite having reached the Western Highlands, I still cannot purchase the relevant speed upgrade.
It's possible that it's locked behind story progression. It's possible that it's locked behind ranking within Clan Centurio which is level locked more than anything. Either way, I fail to see why this would be so.
I request that the map speed upgrades be available as soon as you unlock the map. IIRC The mount speed upgrades affect only ground speed. Once you have flying unlocked, the speed upgrades become redundant. Having them available when you unlock the area will make them actually useful

As for how to get them, well, there's the daily hunt marks, which will be a grind, but here's the bigger thing - ARR hunt marks award you Centurio if you have it unlocked. It's very possible albeit still grindy to have the necessary amount of Centurio seals to have the first speed upgrade ready as soon as you enter any HW area. Given how big they are compared to ARR areas, having the mount speed upgrades available asap will be a large boon.
Admittedly it is kind of roundabout because you can easily unlock flying by the time you actually get the mount speed. But I'm trying to unlock the ARR mount speeds before progressing with HW, and having unlocked Centurio seals, I'm earning them too. Why shouldn't I have access to the buyable mount speed upgrades?