2009 wireless keyboard giving serious lag on macbook pro
Without being cheeky, it could just be a sign of age. The keyboard itself is 9 years old and the MBP is 6 years old.

You did well to get so long out of the keyboard, but I wouldn't be too sure what specifically could be wrong with it or if it would be worth fixing.

I wouldn't rule out a hardware problem caused by 9 years of usage.