The Storm Sorcerous Origin (from Unearthed Arcana) includes the following feature:
Wind Soul
At 18th level, you gain a flying speed of 60 feet…
The Mobile feat (PHB, pg 168) includes the following:
Your speed increases by 10 feet.
If a character had both these traits, would they have a flying speed of 60ft or 70ft?
Similarly, if the sorcerer casts Haste on themselves, would their fly speed increase from 60ft to 120ft? And if Mobile does affect the fly speed, would the Haste spell then increase it to 140ft?
Best Answer
So the question here is:
Do bonuses to 'your speed' apply to flying speed?
Sadly, the answer is that is that it's not clear. I'll break down the arguments for and against.
For
Against
To me, the last two arguments are the strongest ones, since they're the only attempts to actually define what 'your speed' means. That said, it's still a bit hazy, so I guess the answer (as ever is) has to be:
Talk to your DM. It's up to them to decide whether this works or not. I would definitely point out to your DM that this boost of 10 feet is effectively all you'd be getting from the Mobile feat, because:
Feats are meant to be powerful, and if all you're going to be getting is an increase from 60ft -> 70ft, it's not really a big deal. In fact, if I were you I probably wouldn't take Mobile. Haste is trickier, because it's a very small investment that would boost your speed up to 180ft (with the bonus Dash action), making you faster than any creature in the Monster Manual. Of course, it doesn't make much sense to allow one and not the other.