Spirit Guardians interact with the rules in a strange way in relation to the speed of RAW
Speed is defined as:
Each character and monster has a speed, which is the distance in feet the character or monster can travel in one turn. This number is based on short, energetic movements in the midst of a life-threatening situation.
In the case of spirit guards, the effect indicates (emphasize mine):
The speed of an affected creature is halved in the areaand when the creature first enters or begins the area in one turn, it must make a wisdom roll.
While the character is in the area, the speed is halved. If a character normally has a 30 foot speed and moves around, he has a speed of 15 feet while in the area. The same is true for a creature that begins its turn out of range but moves into the area. Their speed is halved in the area.
Take this example:
- Sign A: Usually 30 feet, the arc begins 30 feet from the center of the spell
- Character B: Usually 30 feet, the curve begins within the range, 5 feet from the edge
- Characters C: Usually 30 feet, the curve starts in the middle of the range
Character A can move 15 feet to get to the edge of the spell area, but once he enters the area, his speed is halved. By entering the area they have spent 20 feet of movement, but their speed is 15 feet. As a result, they can not move anymore.
Character B begins at a speed of 15 feet due to the spell. You can spend 5 feet of movement to leave the area. Since they are no longer in the area, their speed is no longer halved. As a result, their speed is now 30 feet, they have spent only 5 feet of it, so they have 25 feet left as they wish.
Character C can move 15 feet to get to the edge of the spell effect, but can not leave it. As a result, they have a speed of 15 feet for their entire turn and their movement ends when they reach the edge of the spell.
Some of them may seem a bit absurd, but that's how the rules read.
Alternative Solution: Treat the area of the spell as difficult terrain for a more consistent application
A more manageable way to treat this spell as DM would be to designate the area in this spell as difficult terrain. In difficult terrain, a creature must spend 10 feet of its speed to move 5 feet. As a result, the time you spend in this area affects all characters, no matter where they begin relative to the center of the spell.
In situations where the spell overlaps with terrain that is already difficult, you should double the difficulty of the terrain within the spell area so as not to affect the spell's effectiveness. So, in the area of the spell, I have to apply a creature at a speed of 4 feet in difficult terrain to move 1 foot. Rangers and druids should not be immune to this change from halving speed to difficult terrain. (Thanks to @PixelMaster and @MarkWells for pointing out these two issues with the exclusion)