The movement reduction percentage is 85 minus twice the creature's strength, with a minimum reduction of 1%. This is calculated upon entering the area of effect and is not affected by changes to a creature's strength while the creature is in the area of effect.
This differs from the web spell in that its DC is fixed, spell resistance does not apply, and (of interest to builders) there is no OnSpellCastAt event. Furthermore, since this is a different spell, standard spiders are subject to it.
Sometimes—due to a bug in the game—the speed reduction is not removed when a creature leaves the area of effect, resulting in a permanent speed decrease.