Does the script for the removal planner run for every single target? Because if so, it's very inefficient for when you deconstruct lots of stuff (say, a couple thousand items that spilled on the ground)

no, it checks the name of the deconstruction planner used so it only runs for the modded planners

Does it do it per item, or per decon swipe? I'm thinking it's the formor. I don't remember having several minutes of slowdown over a modernatly large swipe.

every time you deconstruct, it checks the name of the item, if it is not the correct name it immediately stops, if it is the correct name, it goes over all selected things and adds lights to them

Experiment: disabled all of the hazard light mods, and deconstruct a large chunk of my base. slowdown of about a second.
Enabled Hazard lights and auto lights: slowdown is somewhat longer, and even longer on undoing deconstruct.
Enabled selection tool: Several minutes worth of complete freeze. More than 5 probably.
Edit: closer to 10 now.
Edit edit: possibly 15 now?

I'm about 90% sure that you misunderstand how that event works. since the slowdown also triggers for deconstructs that don't happened due to anyone using the deconstruction planner (AKA deconstructs that happened via script)

it seems like i remembered wrong, it did do that check for every single entity selected

but i have made some changes and hopefully now it should be a bit faster
at least now it does as i remember