Agree with Ian, probably time to clean your biomedia in tank water if your flow rate has slowed. If you need any tips about media cleaning the members can help.
Everyone's answer about turning the filter off would/should be different because filters come in so many different types and configurations. Filters that might get air in the inlet need to be turned off. On weekends when a filter clean is in the schedule, the filter will get turned off in anticipation of that happening after the substrate has been cleaned.
On routine gravel-clean-water-changes without filter cleaning I just leave my filter on, but my inlet is low and my water change process doesn't expose my pumphead to a long time of less incoming pressure. The safe answer of course is just to always turn the filter and heater off, so any beginners having any doubt should just do that.
~~waterdrop~~