Battery usage

Apple has some tips on maximising battery life and lifespan which, as is explained in the first linked document below, are different things. See:

Batteries - Maximizing Performance (scroll down to tips for iOS devices)

iPhone Battery and Performance

Or perhaps the battery in your old iPhone is no longer holding a satisfactory charge and has reached the end of its lifespan. Use the instruction in iPhone Battery and Performance to examine the current Maximum Capacity. If it is under 80% then it's time to consider a new battery. If it needs replacing look at the support document iPhone Battery Replacement

Due to old hardware, (Processor, Ram etc) It will work slow