If you would like the position speed to remain the same but the orientation speed to change then have two different tweens with different time to achieve it. (and loop the faster one until they are both done!)
local timetoComplete = 22
local offset = -100
local spinning = true
local spinSpeed = 1 -- degrees
-- moving the nado
local move = TS:Create(nado, TweenInfo.new(timetoComplete, Enum.EasingStyle.Linear, Enum.EasingDirection.Out, 0, false, 0), {Position = nado.Position * (CFrame.new(0, 0, offset).Position))
move:Play()
move.Completed:Connect(function()
spinning = false
end)
-- spinning the nado in Y direction
task.spawn(function()
while spinning do
nado.Rotation += Vector3.new(0, math.rad(spinSpeed), 0)
task.wait()
end
end)