Stuttering camera movement when using BindToRenderStep on an NPC?

I'm getting into camera manipulation, and with the help of the wiki I made a very simple camera script that tracks an NPC using the BindToRender step function.

local runService = game:GetService("RunService")

local player = game.Players.LocalPlayer
local camera = game.Workspace.CurrentCamera
local cameraSubject = game.Workspace:WaitForChild("NPC")
local offset = Vector3.new(20, 5, 20)


local function onRenderStep()
    local subjectPosition = cameraSubject.Torso.Position
    local cameraPosition = subjectPosition + offset
    camera.CFrame = CFrame.new(cameraPosition, subjectPosition)
end

runService:BindToRenderStep("Camera", Enum.RenderPriority.Camera.Value, onRenderStep)

When I go into a server, if my player moves this character, I experience no "lag" on my screen and the transition between frames is seamless. However, on my test device the camera seemed to only be updating every few seconds. It worked the same way vice versa. What is this caused by, and how can it be resolved?