Why do vacuum tubes need time to warm up before operating?

Vacuum tubes require a warm-up period because they rely on thermionic emission to function. This process involves applying heat to the cathode so that electrons within the material gain enough kinetic energy to overcome the electronic work function and escape the surface.12

Once these electrons are emitted, they can travel to the anode to create an electric current. Because this emission mechanism is dependent on the cathode reaching a specific operating temperature, the tube cannot begin conducting current until it has sufficiently heated up, which typically takes anywhere from a few seconds to several minutes.12