When i spam left shift,it is just really buggy

Help and Feedback Code Review
1 
--Services--
local CAS = game:GetService("ContextActionService")
local RNS = game:GetService("RunService")
--Setting--
local staminaUsage = 0.5
local rechargeTime = 1
--Script--
local character =script.Parent
local humanoid = character:WaitForChild("Humanoid")

local UserInputService = game:GetService("UserInputService")
local player = game.Players.LocalPlayer

local lastSprint = math.huge

local isSprinting = false

local function sprint(actionName , inputState , inputType)
	if inputState == Enum.UserInputState.Begin then
		isSprinting = true 
	elseif inputState == Enum.UserInputState.End then
		isSprinting = false
	end
	if isSprinting then 
		humanoid.WalkSpeed = humanoid:GetAttribute("SprintSpeed")
	else
		humanoid.WalkSpeed = humanoid:GetAttribute("BaseWalkSpeed") 
	end
end

CAS:BindAction("Sprint", sprint,true,Enum.KeyCode.LeftShift)

RNS.RenderStepped:Connect(function(delta)
	local stamina = humanoid:GetAttribute("Stamina")
	if stamina < staminaUsage then
		isSprinting = false
		humanoid.WalkSpeed = humanoid:GetAttribute("BaseWalkSpeed")
	end
	if isSprinting and stamina > 0 and humanoid.MoveDirection.Magnitude > 0 then
		humanoid:SetAttribute("Stamina" , stamina - staminaUsage)
		lastSprint = tick()
	else
		 if tick() - lastSprint>= rechargeTime and stamina< humanoid:GetAttribute("MaxStamina") then 
			humanoid:SetAttribute("Stamina" , stamina + staminaUsage)	
		 end
	end
	UserInputService.InputBegan:Connect(function(input,gameProcessedEvent)
		if UserInputService:IsKeyDown(Enum.KeyCode.LeftShift) then
			for i = 1 , 0 , -0.1 do
				wait(0.1)
				player.PlayerGui.StaminaScreen.StaminaBar.BackgroundTransparency = i
				player.PlayerGui.StaminaScreen.StaminaBar.Remaining.BackgroundTransparency = i
				player.PlayerGui.StaminaScreen.StaminaBar.ImageLabel.ImageTransparency = i
			end
		end
	end)
	repeat wait()
	until stamina == 100
	UserInputService.InputBegan:Connect(function(input,gameProcessedEvent)
		if UserInputService:IsKeyDown(Enum.KeyCode.LeftShift) then
			for i = 1 , 0 , -0.1 do
				wait(0.1)
				player.PlayerGui.StaminaScreen.StaminaBar.BackgroundTransparency = i
				player.PlayerGui.StaminaScreen.StaminaBar.Remaining.BackgroundTransparency = i
				player.PlayerGui.StaminaScreen.StaminaBar.ImageLabel.ImageTransparency = i
			end
		end
	end)
end)
2

I want to make a sprint system when we hold left shift the sprint bar will fade in and it will fade out until the sprint bar is full