How does the movement enhancement from Mobile feat interact with the movement speeds of class and sub-class features? [duplicate]

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.

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How does the Mobile feat interact with the Writhing Tide for a Swarmkeeper Ranger?

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.

Writhing Tide

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?

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