There was a specific hardware / RF design issue with the first-gen left joycons that caused a weak signal and resulted in lag or bad inputs. If you have that on

There was a specific hardware / RF design issue with the first-gen left joycons that caused a weak signal and resulted in lag or bad inputs. If you have that one, get it fixed. Nintendo was (at the time) fixing them all for free. There are also youtube videos of how to do the fix yourself for a couple of bucks.