Slowest speed operation in lathe is Turning.

Reasons:

1. Turning is a basic operation in lathe and is performed on a regular basis. It involves removing material from the workpiece to produce a cylindrical shape. The speed at which the material is removed is relatively slow, making it the slowest speed operation in lathe.

2. Turning involves using a single-point cutting tool to remove material from the workpiece. The tool moves across the workpiece, removing material as it goes. This process can be time-consuming, particularly when removing a large amount of material or working with a hard material.

3. The speed at which the lathe is operated during turning depends on several factors, including the material being worked on, the size of the workpiece, and the type of cutting tool being used. In general, slower speeds are used for harder materials and larger workpieces.

4. Turning is a relatively simple operation compared to other lathe operations, such as thread cutting, taper turning, and knurling. These operations require more complex tooling and techniques, which can result in higher speeds and more rapid material removal.

Conclusion:

In conclusion, turning is the slowest speed operation in lathe due to its relatively simple nature and the slow speed at which material is removed from the workpiece. While it may be time-consuming, it is an essential operation for producing cylindrical shapes and is performed on a regular basis in many industries.