Clogged Catalytic Converter Nissan Altima

How to diagnose a clogged Catalytic Converter using one of my favorite tools. inexpensive but very useful Vacuum Gauge. The Car is a 2006 Nissan Altima with a QR 25 DE as an example. The Car would not rev past 3500 RPM. and had poor acceleration and no hill climbing ability. MPG was around 18 MPG highway And rose to 26 MPG highway after the repair. Here is the link to the New Catalytic Converter amzn.to/3qmCMGl and to the vacuum guage amzn.to/3gMmXE9