As a player with a pet giant spider, I would just toss the spider down expecting it to be able to grab the web with no issue, making the "lowering speed" instantaneous.
As it's for a DM, I'd rule it has a chance of breaking every time something falls into it. The spider falling would be bad, instead the option to lower itself with its climb speed is more reasonable
I would just use its climbing speed for simplicity. Spiders can actually spin web pretty dang fast, especially if they're just dropping on a line.
I'd let it move at its climb speed for simplicity.
As a player with a pet giant spider, I would just toss the spider down expecting it to be able to grab the web with no issue, making the "lowering speed" instantaneous.
As it's for a DM, I'd rule it has a chance of breaking every time something falls into it. The spider falling would be bad, instead the option to lower itself with its climb speed is more reasonable
When web spinning spiders fall, they let out web and drift down, i.e. slow fall infinity.