Major issue lol short circuit or too thin of wire/breaker, old house probably. Instant dim and back to normal turning on a heavy appliance can happen as the power circuit lags but it’s a mere fraction of a second.

So to turn on an appliance I’m pretty sure it takes 3000 watts to cycle on then reduces to say 1500 watts to operate a normal 1500w appliance. Nothing should ever continuously dim lights. Major fire hazard if so.