To fall behind in movement, progress, or development. The runner began to lag behind the rest of the group during the final lap.
To experience a noticeable delay in response or performance, especially in computing or gaming contexts. The video began to lag during the livestream because of the poor connection.
To fail to keep up with a schedule or deadline. The project started to lag due to unforeseen complications.
Chiefly British: to cover or line (a pipe, boiler, etc.) with insulating material. The engineer lagged the hot-water pipes to save energy during the winter.