In the Swarmkeeper section for Rangers in Tasha’s Cauldron of Everything there is an ability called Writing Tide (TCoE, p.60), which allows the Ranger to fly.
7th-level Swarmkeeper feature
You can condense part of your swarm into a focused mass that lifts you up. As a bonus action, you gain a flying speed of 10 feet and can hover. The effect lasts for 1 minute.
You can use this feature a number of times equal to your proficiency bonus, and you regainall expended uses when you finish a long rest.
In the Player’s Handbook the feat Mobile (PHB, p.168).
You are exceptionally speedy and agile. You gain the following benefits:
• Your speed increases by 10 feet.
• When you use the Dash action, difficult terrain doesn’t cost you extra movement on that turn.
• When you make a melee attack against a creature, you don’t provoke opportunity attacks from that creature for the rest o f the turn, whether you hit or not.
Does this mean that if you take Mobile as a feat, the benefit would apply to this particular feature too – effectively having a fly movement of 20 feet instead?