Historically low interest rates are starting to take a toll throughout the financial industry, presenting a potential downside to the Federal Reserve’s aggressive efforts to reignite growth in the sluggish economy. Rock-bottom rates are squeezing profit margins at banks that rely on the gap between what they charge borrowers and pay depositors. They also are hurting returns at pension funds already under mounting pressure to meet obligations to retirees, while making certain kinds of insurance more expensive as firms try to recoup earnings that are likely to shrink if the ultralow rates linger.