The Mobile feat appears in the Player’s Handbook (PHB, p.168) and would be generally accessible to all classes and races as long as DM agrees to allow feats at the table.
You are exceptionally speedy and agile. You gain the following benefits:
• Your speed increases by 10 feet.
There are certain class or sub-class features that allow a PC to have increase in movement (walking) such as: a Barbarian’s Fast Movement (+10 feet), a Monk’s Unarmored Movement (+10 to +30 feet), or a Druid’s Wildshape (walking, depending on the beast), a Scout Rogue’s Skirmisher (reaction, moves half their speed).
There are also certain class or sub-class features that modify existing or provide additional movement options other than walking, such as: Swarmkeeper Ranger’s Writhing Tide (10 feet flying), Twilight Cleric’s Steps of the Night (10 feet flying), or a Ranger’s Deft Explorer (+5 feet; climbing/swimming speed sam as walking), a Druid’s Wildshape (swimming/flying, depending on the beast), Phantom Rogue’s Ghost Walk (10 feet flying), The Fathomless Warlock’s Gift of the Sea (40 feet swimming).
I’m looking for an answer for this specific question with relation to how the Mobile feat interacts with class/sub-class features which provide adjustments to movement speeds.
From what I gather all these class/sub-class features would add a +10 feet movement, e.g. a Warlock with the The Fathomless sub-class who takes the Mobile feat would have a 50 feet swimming speed.
Is this correct?
Thanks for the clarification/confirmation on this.