What does it take to get out of Grease and Web at the same time?

Question

Last night in my session, a wizard and I stopped a small group of pursuers chasing our wagon with a timed Grease and Web spell cast at the same time. With the group failing at least one of the saves, it turned the potential combat into just straight up negotiations.

Not every encounter may be ended directly by this though so I'm curious how these two spells interact with someone failing at least one and in the intersections of the two spells. Once caught in a greasy web, how hard is it to get out and what/how many successful or additional saves need to be made? Being restrained and prone seems to leave very little options and with the spell saves needing to be repeated at certain points it seems like the checks compound quickly.

For simplicity, I'll also put the spell descriptions below:

Web:

You conjure a mass of thick, sticky webbing at a point of your choice within range. The webs fill a 20-foot cube from that point for the Duration. The webs are Difficult Terrain and lightly obscure their area.

If the webs aren't anchored between two solid masses (such as walls or trees) or layered across a floor, wall, or ceiling, the conjured web collapses on itself, and the spell ends at the start of your next turn. Webs layered over a flat surface have a depth of 5 feet.

Each creature that starts its turn in the webs or that enters them during its turn must make a Dexterity saving throw. On a failed save, the creature is Restrained as long as it remains in the webs or until it breaks free.

A creature Restrained by the webs can use its Actions to make a Strength check against your spell save DC. If it succeeds, it is no longer Restrained.

The webs are flammable. Any 5-foot cube of webs exposed to fire burns away in 1 round, dealing 2d4 fire damage to any creature that starts its turn in the fire.

Grease:

Slick grease covers the ground in a 10-foot square centered on a point within range and turns it into Difficult Terrain for the Duration.

When the grease appears, each creature standing in its area must succeed on a Dexterity saving throw or fall prone. A creature that enters the area or ends its turn there must also succeed on a Dexterity saving throw or fall prone.

Answer 1

Of course all characters that fail their save against the web are stuck in it.

Once a character saves against the web, how hard is it to leave the area?

A web is a 20-foot cube, and a grease is a 10-foot square. If you're not playing on a grid, you could get a situation where a character is at the exact center of the web and the grease, so they would need to move 5 feet to exit the grease and an additional 5 feet to exit the web.

Here are some relevant rules about being prone:

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

To move while prone, you must crawl or use magic such as teleportation. Every foot of movement while crawling costs 1 extra foot. Crawling 1 foot in difficult terrain, therefore, costs 3 feet of movement.

So, if a character is prone but unrestrained in the position above, one option would be to stand up (using 15 feet of movement) and then walk 7.5 feet over difficult terrain (using 15 feet of movement), ending in the web but not in the grease. The character would need a new save vs web on the next turn.

Another option would be to simply crawl out of the area, using 30 feet of movement to move 10 feet through difficult terrain. The character would end up exactly on the edge of the web, and it's likely that the DM would rule that the character would still need a save against the web on their next turn.

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If you are playing on a grid, then you're going to get something like this:

. . . . . . . .
. . w w w w . .
. . w G G w . .
. . w G @ w . .
. . w w w w . .
. . . . . . . .

and the grid rules tell us that you pay additional movement for moving into a square with difficult terrain. So in this case you could stand up (costing 15ft of movement) and then move two squares south (costing 10ft of movement for the first and 5ft of movement for the second). Or you could crawl two spaces south, costing 15ft of movement for the first and 10ft of movement for the second. Standing up first is the better option.

Answer 2

It is an interesting low level control combination. In addition to the initial effects, as a DM I would probably compound the difficulty - assuming the pursuer made the web check but failed the grease check, momentum would have them falling and likely sliding further into the web. I would probably have required a second Web save in that instance for monsters/npcs to reward the creativeness of the combination.

Once prone, crawling is an option to avoid the slip/fall grease save, but does not negate the web save. In addition, assuming a creature is in the center of the 10' cube of web, that leaves 10' of crawling over difficult terrain. So total movement to make that in a single round to avoid another web save would be 10' x 2 (difficult terrain) x 2 (crawl speed), right? So, more than 40' of movement to clear the web in 1 round.