You might find this is Unity auto generating the lighting data for your games.
To test the theory, goto the menu at the top, select Windows, then Rendering and Lighting Settings.
The Lighting Settings window will appear, you can dock it like any of the other windows if you want to. At the bottom of the settings you’ll see an option for Auto Generate, untick it.
This will prevent Unity from generating you lighting data each time you. Modify something in your scene. This process can be resource intensive, so if you have a lower spec computer you may experience this as lag.
Remember of course that with this option disabled Unity is not automatically generating your lighting data, although this will be done when you build. The down side of this is that you don’t get a flavour for how things will look in the scene whilst you are working away.
What you can do though is use the Gemerate button which is next to the Auto Generate option to manually create the lighting data. You could do this at the beginning and then just periodically as you make significant changes within the scene.
Remember, this option and the data is scene soecific, so you will need to redo these steps in others scenes you work in also.
Hope this helps