Really worn turbos also don't build boost effectively. They use exhaust gas to pressurize the intake air. F50PL002C, F51CAFSR015B, F51CAYSR037, 9VA03, 9VA14.
I bought a used Audi A4 with an underperforming turbo. I lived with it for a year and finally decided to replace it. Wow! Now I know what kind of power a turbocharged car should have. I should've done this sooner!