Does standing up from being prone in difficult terrain consume all of your movement?

Question

The rules for Difficult Terrain stipulate:

You move at half speed in difficult terrain— moving 1 foot in Difficult Terrain costs 2 feet of speed

while rules for standing up from prone:

Standing up takes more effort; doing so costs an amount of movement equal to half your speed.

Is the act of standing up RAW "movement", such that in difficult terrain, it costs the entirety of your movement? And, would that mean that reducing a creature's speed (with a spell like Slow, for example) precludes them from being able to stand up in this situation?

Answer 1

Difficult terrain does not affect standing up from prone.

The rules for difficult terrain state:

You move at half speed in difficult terrain— moving 1 foot in Difficult Terrain costs 2 feet of speed

"Move" and "moving" here are referring to the rules for movement and position:

On your turn, you can move a distance up to your speed. You can use as much or as little of your speed as you like on your turn, following the rules here.

Your movement can include jumping, climbing, and swimming. These different modes of movement can be combined with walking, or they can constitute your entire move. However you're moving, you deduct the distance of each part of your move from your speed until it is used up or until you are done moving.

The distance travelled when standing up from prone is 0 feet, so you "move" 0 feet when standing up from prone. Since you move 0 feet when standing up from prone, difficult terrain does not impose any additional penalties when standing up.

It is worth noting that this question was answered in version 1.14 of the Sage Advice Compendium, but was removed without comment for version 2.0:

Is standing up from prone considered moving? Standing up costs movement but moves you nowhere. When the game refers to you moving, it means moving some distance. It doesn’t mean making a gesture or standing up in place. To move while prone, you crawl or use magic (PH, 191).

Answer 2

Standing up is not movement; it is a value change

The wording of standing up is "costs an amount of movement equal to half your speed". So the character is not going anywhere, this just deducts the amount of distance they can travel.

This works similar to the Sentinel feat:

When you hit a creature with an opportunity attack, the creature's speed becomes 0 for the rest of the turn.

The feat changes the distance the target can travel in a turn--in this case 0. Mind you, this change to 0 can come after not moving at all, or after walking 20 ft. The feat just changes movement to a set value regardless of the current value.