Why is the energy-saving lamp only half lit when it is first turned on, and then it is all lit?

There are three possibilities. If it's the first time to use it, it's probably because the lamp is not excellent when it leaves the factory and the fluorescent lamp is not fully activated. It will take some time for the lamp to be activated after heating up.

In cold weather, the air pressure (pipe pressure) in the lamp tube is low when it is started, and it needs to wait for the lamp tube to heat up before it can be normal.

The solid mercury lamp tube itself needs more time than the ordinary lamp tube to make the air pressure in the lamp tube reach the best working state.

In fact, all fluorescent lamps have this feature, but it is not obvious when the weather is not very cold.