Stormborn feature, what does a character's "current walking speed" account for?

[Aidamis]

Hello folks.

I was reading the Tempest Cleric description (it's for a BBEG) and noticed the "flying speed equal current walking speed" rule.

While I'm pretty sure that means that a Human TC with Mobile has a flying speed of 40 ft/round, what about one that cast Longstrider on itself? does the speed become 50 ft?

What about additional effects such as potions?

Thank you very much for your input.

[CheddarChampion]

Things that count:
Mobile +10
Wood Elf +5
Centaur +10
Small race (or dwarf) reduction -5
Squat Nimbleness +5
Longstrider +10(?)
Haste (double base)
Barbarian's fast movement +10
Monk's unarmored movement +X
(Probably some magic items that directly boost speed)

Things that don't count:
Tabaxi's "feline agility"
Orc's "aggressive"
Rogue's cunning action dash
Monk's step of the wind
Aarakocra's flight speed
Winged Tiefling's flight speed
Winged Boots
The spell "Fly"
Riding a mount

[Aidamis]

Many thanks for the exhaustive list.
Now I'm kinda leaning towards building a Dragon-ogre Shaggoth-like centaur. And if that thing can also fly, oh boy.