How to lag efficiently check if another part is above my part?

So basically i already made a solution for that with the problem being.. It's REALLY laggy.

And i kind of understand why it does that but it's the only solution i know of.

Example of what i'm using:

for _,v in pairs(workspace:GetChildren()) do
if v:IsA("BasePart") then
if (v.Position - MyPart.Position + Vector3.new(0,2,0)).Magnitude < 5 then
myfunction()
end
end
end

now it does work but like i said it's very laggy!

I've been thinking about using rays but i'm new to them since i never bothered trying them out in any of my scripts but i think they can stop at certain points when they hit a part if you use the right script.

Any ideas on how i could fix this?