Is the additional movement granted by the Mobile feat also halved when a character stands from prone?

Question

The game essentially packages a character's maximum velocity, acceleration potential, and agility into a commodity called "speed". This makes sense because these attributes are impossible to meaningfully compare across a variety of creatures of different sizes, strengths, body configurations, etc.

The Mobile feat (PHB, p. 168) essentially adds 10 "speed points" to a character's movement budget to reflect an increase in all movement abilities.

Thus a fighter with a pre-feat budget of 30 "speed points" must spend 15 (50%) of those to stand from prone. The question is: does it still cost him 15 "speed points" to stand post-feat? Or must he now spend 20 "speed points" (5 more) to do the exact same thing (more slowly) when, post-feat, he is presumed faster and more agile?

Answer 1

Yes, it's halved

Mobile: [...] Your speed increases by 10 feet.

And

Prone: [..] Standing up [from prone] takes more effort; doing so costs an amount of movement equal to half your speed. For example, if your speed is 30 feet, you must spend 15 feet of movement to stand up.

So mobile increases your speed, and prone costs you half your speed. If you have 30 speed and add 10 from mobile you get 40, if you stand up from prone that turn you have 20 left since standing up costed you 20. Nothing contradicts this general ruling, so it's RAW and (most likely) RAI.

If you have a hard time wrapping your head around this think it this way: mobile allows you to run faster, but not to move faster. So when you are standing up you still consume half your movement to stand up, since you can't stand up faster than someone without mobile, but once you stand up and start running you still can run faster than someone without the feat since your running speed is higher, just not as much distance as normally since you only have half the time to run.

However, as @PixelMaster said, you could get the Athlete feat to reduce the cost of standing up from prone to only 5 feet of movement, thus benefitting from the full 10 speed of the Mobile feat.

Answer 2

tl;dr: Yes.

It's really easy. The mobile feat increases your speed by 10 feet, so if your base movement was 30, it is now 40. Then, if you stand up, you use half your movement. The rules are very clear on that.

The reasoning behind this rule, instead of a fixed value, might be to ensure that high-speed characters still feel the effects of falling prone, aside from the advantage against them by melee attacks. Also, it prevents slow races like dwarves from being unable to get up if a spell reduces their speed by another e.g. 10 feet. Being prone is a really annoying condition if you have melee-based opponents.

However, you could just take the “Athlete“ feat - it reduces the cost of standing up to 5 feet.

Answer 3

Yes, it does. Standing up from prone costs you half your movement, and Mobile increases your movement.

It's not all that counter-intuitive though; Mobile doesn't make you faster at standing up, so you spend half of your turn standing up and then the other half of your time moving at Mobile's higher base speed. The end result is moving half the distance you'd normally move.

Answer 4

Yes

The feat increases the movement speed. Think of it as a modifier to your movement. The simplest comparison is Resistance and Vulnerability. If this was damage you resisted and their attack did 10 and had a +4 damage modifier, you would take (10+4)/2 damage... or 7 damage. It's the same concept, just applied to a different mechanic.

Answer 5

Yes, that is how the rules, as written, work. Faster creatures take more movement to stand up.

It doesn't, as you have noticed, make full rational sense. And the attempts to make sense of it are doomed to failure, because it just doesn't make full sense. But that's how it is.

It is a minor issue with regard to the game, and game balance, so I wouldn't worry about it too much. Either live with the rule as it is, to make it easy and keep your game consistent with other games and the RAW, or make a new rule.

I might, if I found this bugged me enough, make a rule to say that all Medium creatures take 15' of movement to stand, and Small creatures 10' of movement. This would make for a minor advantage to gnomes and halflings, and a minor disadvantage to dwarves, but nothing that would break the game.