Conveniently Slow

"It's a real relief to see this bomb to blow the whole Earth up won't detonate until next Thursday, giving me enough time to figure out how to cut its wires."

When something is Conveniently Slow, it's just slow enough to "pose" a "threat" to whoever it's endangering without actually hurting them as their savior was given enough time to rescue them. A well-known example of this is the slowly-moving conveyor belt gradually forcing the tied-down victim to their death that's on the other side of the belt.

If the Villain is going to kill the Hero's friends with some slow method, like lowering them into acid from cages, with bombs that have timers, or filling a closed room with water, this would have Real Logic Question Fiction with "If you want to kill your enemy's allies, why not immediately do it?".

Conveniently Slowly advancing to the end of an evil plan will allow a Dramatic Slow Motion moment when the Cavalry finally arrives at the scene to Save The Day, often with them reaching their hand out to some device like a button or lever that can stop a machine from finishing the job, a Countdown Timer that says the plan will be completed Mere Moments Away, and/or with the camera cutting to everyone's faces filling with suspense as they watch the Cavalry save their friends.

Conveniently Slow as an UnTrash gets poor reception. Having the Villain execute their plan so sluggishly when they're just a hair's breadth away from finally succeeding for once would have some audience members roll their eyes in annoyance at the stupidity of the idea.