2008 Cadillac CTS Power Steering

You could have one of two problems. First, a weak power steering pump could cause the concern. Second would be the steering gear assembly itself bypassing internally.
If you lose pressure, it will be difficult to turn at low speeds. This is due to the weight of the vehicle. You will notice it takes much less effort to turn the wheels when the vehicle is in motion. The faster you go, the easier it is to turn.
The power steering pump increases assist pressure at lower speeds and decreases at higher speeds. You need to make sure the pump is able to build pressure. To do this, you will need a pressure gauge connected to the high pressure line at the pump. It does take a special tool to do this. This tool is used by the GM dealership service center to measure output pressure and flow rate of the power steering system.
I would recommend having the system tested at your local dealership to avoid replacing parts that are working properly.

Power Steering Pump Specifications
Flow 2.8 GPM (gallons per min)
Pressure relief 1624/1740 PSI (pounds per square inch)

I have included a picture of the system with the tool connected to give you an idea of how it operates.