How does Freedom of Movement interact with magical effects which increase movement costs?

Question

The text for the Freedom of Movement spell contains the following clause:

the target's movement is unaffected by difficult terrain, and spells and other magical effects [cannot] reduce the target's speed

While this is fairly clear with regards to spells which produce difficult terrain (such as Entangle) or spells which reduce speed (such as Spirit Guardians), it is less certain for spells which increase movement costs. Consider for example the text for the fog produced by the Druid Grove spell:

every foot of movement through the fog costs 2 extra feet

Similarly, consider the text for the Etherealness spell:

If you move up or down, every foot of movement costs an extra foot.

Both of these spells increase the amount of speed consumed when moving. Does Freedom of Movement allow the target to bypass these additional costs? Or does it do nothing as it only prevents speed “reductions”?

Answer 1

It depends on how you read "speed"

Just like hit points, speed is an abstract value used in order to simplify an aspect of a character, in order to more easily interact with them. In this case, speed represents a character's ability to move a certain distance during a single unit of time.

As such, there are two ways to define what it means to "reduce the target's speed", and it all comes down to how we read the word "speed" in this context.

Down to numbers

We can see "speed" as nothing more than the number on a character's sheet. A key/value relation, where the word "speed" refers specifically to the number itself. As such, anything that alters that number is considered as a reduction to speed.

With this interpretation, effects that make you use more of your speed to move around are not a reduction to your speed, since they do not directly interact with the "speed" value of a character.

As such, Freedom of Movement would not help with those effects.

Up to the abstracted concept itself

An effect that makes you spend more speed to move around is effectively an effect that reduces your ability to move around. As such, while it does not reduce the number called "speed" directly, such an effect does reduce your effective speed.

With that interpretation, Freedom of Movement would protect against those effects.

Discuss it between DM and player

Depending on what kind of enemies or environments the DM prepares for their players, either of those interpretations could have major consequences on the balance of upcomming battles.

Because of the ambiguous nature of this debate, your best bet is to discuss it with your DM (or the concerned player, if you are the DM) and decide on which ruling to make, so that there is no bad surprise for either side during a fight.

Answer 2

It doesn't by RAW

Freedom of Movement only protects from Difficult Terrain and reductions to movement speed. The increased cost of movement from Grove and Etherealness is neither. Spells only do what they say they do and all that.

However

Eddymage's reasoning is fair and ruling that Freedom of Movement also protects from these kinds of effects is fine.

Answer 3

Increasing movement costs means reducing your speed: Freedom of Movement protects from this.

Recall the rules for difficult terrain (emphases mine):

You move at half speed in difficult terrain — moving 1 foot in difficult terrain costs 2 feet of speed — so you can cover only half the normal distance in a minute, an hour, or a day.

The above rule can be read also as

moving one foot in difficult terrain costs 1 extra foot

and this is confirmed by the rules of difficult terrain in the Combat section of the PHB:

Every foot of movement in difficult terrain costs 1 extra foot. This rule is true even if multiple things in a space count as difficult terrain.

Analyze now the quoted bit from Etherealness spell:

If you move up or down, every foot of movement costs an extra foot.

Under the above interpretation, the up-down movement in the Ethereal plane satisfies the definition of difficult terrain: hence Freedom of movement allows to move at normal cost, per description. Nonetheless, a strict reading of the rules tells that moving up or down in the Ethereal plane does not count as difficult terrain, since it is not explicitly stated.

The Druid's Groove spell says:

every foot of movement through the fog costs 2 extra feet

which can be interpreted as

your speed becomes 1/3 of your normal speed.

meaning that your speed is reduced while in the fog : then, the second property of the Freedom of Movement spell should apply.

In the same way, walking up or down in the Ethereal plane can be seen as moving at half of your speed.