Performance testing Roblox's new OverlapParams functions.

GetPartBoundsInBox.lua

local Workspace = game:GetService("Workspace")

local PART_SIZES = { 0, 25, 100, 1000, 5000, 10000 }

local hitbox = Workspace:FindFirstChild("Hitbox")
local hitboxCf = hitbox.CFrame
local hitboxSz = hitbox.Size

local start = hitbox.CFrame * CFrame.new(hitbox.Size / 2)
local finish = hitbox.CFrame * CFrame.new(-hitbox.Size / 2)

local overlap = OverlapParams.new()

local region = Region3.new(start.Position, finish.Position)

local random = Random.new()
for _, size in ipairs(PART_SIZES) do
	local descendants = {}

	for _ = 1, size do
		local newPart = Instance.new("Part")
		newPart.CFrame = hitbox.CFrame * CFrame.new(
			hitbox.Size.X * random:NextNumber(-0.5, 0.5),
			hitbox.Size.Y * random:NextNumber(-0.5, 0.5),
			hitbox.Size.Z * random:NextNumber(-0.5, 0.5)
		)
		newPart.Anchored = true
		newPart.Size = Vector3.new(2, 2, 2)
		newPart.Transparency = 0.5
		newPart.Color = Color3.fromRGB(255, 255, 255)
		newPart.Parent = Workspace

		table.insert(descendants, newPart)
	end
	
	for i = 1, 2 do
		local useNewApi = i % 2 == 0
		
		-- small wait before we perform benchmark for any expensive operations to finish
		task.wait(2)
		
		-- perform benchmark
		local start = os.clock()
		
		for _ = 1, 100 do
			if useNewApi then
				Workspace:GetPartBoundsInBox(hitboxCf, hitboxSz, overlap)
			else
				Workspace:FindPartsInRegion3(region)
			end
		end
		
		print(size, string.format("%0.4fms", (os.clock() - start) * 1000), useNewApi)
	end

	-- GC instances
	for _, part in ipairs(descendants) do
		part.Parent = nil
	end
	task.wait(5)
end

GetPartBoundsInBox.txt

Similar format results to before, where the first number indicates the amount of parts, second is the amount of milliseconds for 100 API calls, third element is if GetPartBoundsInBox is used (true) or FindPartsInRegion3 is used (false):

0 0.1535ms false
0 0.3553ms true

25 0.1393ms false
25 1.7630ms true

100 0.0809ms false
100 5.3808ms true

1000 0.0638ms false
1000 61.0919ms true

5000 0.0899ms false
5000 514.3444ms true

10000 0.0638ms false
10000 1259.1286ms true

GetPartBoundsInBoxFiltering.lua

local Workspace = game:GetService("Workspace")

local PART_SIZES = { 0, 25, 100, 1000, 5000, 10000 }

local hitbox = Workspace:FindFirstChild("Hitbox")
local hitboxCf = hitbox.CFrame
local hitboxSz = hitbox.Size

local start = hitbox.CFrame * CFrame.new(hitbox.Size / 2)
local finish = hitbox.CFrame * CFrame.new(-hitbox.Size / 2)

local overlap = OverlapParams.new()
overlap.FilterType = Enum.RaycastFilterType.Whitelist

local region = Region3.new(start.Position, finish.Position)

local random = Random.new()
for _, size in ipairs(PART_SIZES) do
	local descendants = {}

	for _ = 1, size do
		local newPart = Instance.new("Part")
		newPart.CFrame = hitbox.CFrame
			* CFrame.new(
				hitbox.Size.X * random:NextNumber(-0.5, 0.5),
				hitbox.Size.Y * random:NextNumber(-0.5, 0.5),
				hitbox.Size.Z * random:NextNumber(-0.5, 0.5)
			)
		newPart.Anchored = true
		newPart.Size = Vector3.new(2, 2, 2)
		newPart.Transparency = 0.5
		newPart.Color = Color3.fromRGB(255, 255, 255)
		newPart.Parent = Workspace

		table.insert(descendants, newPart)
	end

	-- add a random 25 parts to our FilterDescendantsInstances
	local filterDescendants = table.create(#descendants)
	for i = 1, math.min(size, 25) do
		table.insert(filterDescendants, descendants[i])
	end
	overlap.FilterDescendantsInstances = descendants

	for i = 1, 2 do
		local useNewApi = i % 2 == 0

		-- small wait before we perform benchmark for any expensive operations to finish
		task.wait(2)

		-- perform benchmark
		local start = os.clock()

		for _ = 1, 100 do
			if useNewApi then
				Workspace:GetPartBoundsInBox(hitboxCf, hitboxSz, overlap)
			else
				Workspace:FindPartsInRegion3(region)
			end
		end

		print(size, string.format("%0.4fms", (os.clock() - start) * 1000), useNewApi)
	end

	-- GC instances
	for _, part in ipairs(descendants) do
		part.Parent = nil
	end
	task.wait(5)
end

GetPartBoundsInBoxFiltering.txt

Similar format results to before, where the first number indicates the amount of parts, second is the amount of milliseconds for 100 API calls, third element is if GetPartBoundsInBox is used alongside FilterDescendantsInstances set to 25 parts (true) or FindPartsInRegion3 is used (false):

0 0.0708ms false
0 0.3183ms true

25 0.0760ms false
25 2.4035ms true

100 0.0624ms false
100 7.2634ms true

1000 0.1041ms false
1000 81.3375ms true

5000 0.0742ms false
5000 765.0460ms true

10000 0.0678ms false
10000 1717.8362ms true

GetPartBoundsInBoxMaxParts.lua

local Workspace = game:GetService("Workspace")

local PART_SIZES = { 0, 25, 100, 1000, 5000, 10000 }

local hitbox = Workspace:FindFirstChild("Hitbox")
local hitboxCf = hitbox.CFrame
local hitboxSz = hitbox.Size

local start = hitbox.CFrame * CFrame.new(hitbox.Size / 2)
local finish = hitbox.CFrame * CFrame.new(-hitbox.Size / 2)

local overlap = OverlapParams.new()
overlap.MaxParts = 1

local region = Region3.new(start.Position, finish.Position)

local random = Random.new()
for _, size in ipairs(PART_SIZES) do
	local descendants = {}

	for _ = 1, size do
		local newPart = Instance.new("Part")
		newPart.CFrame = hitbox.CFrame
			* CFrame.new(
				hitbox.Size.X * random:NextNumber(-0.5, 0.5),
				hitbox.Size.Y * random:NextNumber(-0.5, 0.5),
				hitbox.Size.Z * random:NextNumber(-0.5, 0.5)
			)
		newPart.Anchored = true
		newPart.Size = Vector3.new(2, 2, 2)
		newPart.Transparency = 0.5
		newPart.Color = Color3.fromRGB(255, 255, 255)
		newPart.Parent = Workspace

		table.insert(descendants, newPart)
	end

	for i = 1, 2 do
		local useNewApi = i % 2 == 0

		-- small wait before we perform benchmark for any expensive operations to finish
		task.wait(2)

		-- perform benchmark
		local start = os.clock()

		for _ = 1, 100 do
			if useNewApi then
				Workspace:GetPartBoundsInBox(hitboxCf, hitboxSz, overlap)
			else
				Workspace:FindPartsInRegion3(region)
			end
		end

		print(size, string.format("%0.4fms", (os.clock() - start) * 1000), useNewApi)
	end

	-- GC instances
	for _, part in ipairs(descendants) do
		part.Parent = nil
	end
	task.wait(5)
end

GetPartBoundsInBoxMaxParts.txt

Similar format results to before, where the first number indicates the amount of parts, second is the amount of milliseconds for 100 API calls, third element is if GetPartBoundsInBox is used alongside MaxParts set to 1 (true) or FindPartsInRegion3 is used (false):

0 0.1057ms false
0 0.9415ms true

25 0.1370ms false
25 1.1478ms true

100 0.0670ms false
100 2.0912ms true

1000 0.0869ms false
1000 19.3874ms true

5000 0.0662ms false
5000 103.2925ms true

10000 0.2380ms false
10000 213.2466ms true

GetPartsInPart.lua

local Workspace = game:GetService("Workspace")

local PART_SIZES = { 0, 25, 100, 1000, 5000, 10000 }

local hitbox = Workspace:FindFirstChild("Hitbox")

local start = hitbox.CFrame * CFrame.new(hitbox.Size / 2)
local finish = hitbox.CFrame * CFrame.new(-hitbox.Size / 2)

local overlap = OverlapParams.new()

local region = Region3.new(start.Position, finish.Position)

local random = Random.new()
for _, size in ipairs(PART_SIZES) do
	local descendants = {}

	for _ = 1, size do
		local newPart = Instance.new("Part")
		newPart.CFrame = hitbox.CFrame * CFrame.new(
			hitbox.Size.X * random:NextNumber(-0.5, 0.5),
			hitbox.Size.Y * random:NextNumber(-0.5, 0.5),
			hitbox.Size.Z * random:NextNumber(-0.5, 0.5)
		)
		newPart.Anchored = true
		newPart.Size = Vector3.new(2, 2, 2)
		newPart.Transparency = 0.5
		newPart.Color = Color3.fromRGB(255, 255, 255)
		newPart.Parent = Workspace

		table.insert(descendants, newPart)
	end
	
	for i = 1, 2 do
		local useNewApi = i % 2 == 0
		
		-- small wait before we perform benchmark for any expensive operations to finish
		task.wait(2)
		
		-- perform benchmark
		local start = os.clock()
		
		for _ = 1, 100 do
			if useNewApi then
				Workspace:GetPartsInPart(hitbox, overlap)
			else
				Workspace:FindPartsInRegion3(region)
			end
		end
		
		print(size, string.format("%0.4fms", (os.clock() - start) * 1000), useNewApi)
	end

	-- GC instances
	for _, part in ipairs(descendants) do
		part.Parent = nil
	end
	task.wait(5)
end

GetPartsInPart.txt

The first number indicates the amount of parts, second is the amount of milliseconds for 100 API calls, third element is if GetPartsInPart is used (true) or FindPartsInRegion3 is used (false):

0 0.0778ms false
0 0.4018ms true

25 0.0913ms false
25 6.1577ms true

100 0.0825ms false
100 21.1336ms true

1000 0.0768ms false
1000 240.5298ms true

5000 0.0632ms false
5000 1459.7305ms true

10000 0.0665ms false
10000 3090.1008ms true

NewApiComparison.lua

local Workspace = game:GetService("Workspace")

local PART_SIZES = { 0, 25, 100, 1000, 5000, 10000 }

local hitbox = Workspace:FindFirstChild("Hitbox")
local hitboxCf = hitbox.CFrame
local hitboxSz = hitbox.Size
local r = hitboxSz.X
local hitboxPos = hitboxCf.Position

local overlap = OverlapParams.new()

local random = Random.new()
for _, size in ipairs(PART_SIZES) do
	local descendants = {}

	for _ = 1, size do
		local newPart = Instance.new("Part")
		newPart.CFrame = hitbox.CFrame
			* CFrame.new(
				hitbox.Size.X * random:NextNumber(-0.5, 0.5),
				hitbox.Size.Y * random:NextNumber(-0.5, 0.5),
				hitbox.Size.Z * random:NextNumber(-0.5, 0.5)
			)
		newPart.Anchored = true
		newPart.Size = Vector3.new(2, 2, 2)
		newPart.Transparency = 0.5
		newPart.Color = Color3.fromRGB(255, 255, 255)
		newPart.Parent = Workspace

		table.insert(descendants, newPart)
	end

	for i = 1, 2 do
		local useNewApi = i % 2 == 0

		-- small wait before we perform benchmark for any expensive operations to finish
		task.wait(2)

		-- perform benchmark
		local start = os.clock()

		for _ = 1, 100 do
			if useNewApi then
				Workspace:GetPartBoundsInRadius(hitboxPos, r, overlap)
			else
				Workspace:GetPartBoundsInBox(hitboxCf, hitboxSz, overlap)
			end
		end

		print(size, string.format("%0.4fms", (os.clock() - start) * 1000), useNewApi)
	end

	-- GC instances
	for _, part in ipairs(descendants) do
		part.Parent = nil
	end
	task.wait(5)
end

NewApiComparison.txt

I ran these on a Hitbox with size 50, 50, 50
Similar format results to before, where the first number indicates the amount of parts second is the amount of milliseconds for 100 API calls third number is if GetPartBoundsInRadius is used (true) or GetPartBoundsInBox is used (false):

0 0.5943ms false
0 0.8691ms true

25 2.1426ms false
25 2.3293ms true

100 5.0867ms false
100 4.4336ms true

1000 47.9035ms false
1000 34.5973ms true

5000 606.7480ms false
5000 637.8543ms true

10000 1367.7854ms false
10000 997.0864ms true