Ifixit battery affecting performance at low power?

Hi All,

I replaced the original battery in my iPhone 6 Plus with an ifixit one and its like a new phone again. All the laggy performance issues I had have completely gone. Mostly.

When the phone is on very low power under 5% the performance falls through the floor; frames missing from animations, apps taking 30-40 seconds to open up then crashing and finally reboot cycles.

Is there a reason for this?

When replacing the battery initially I had no idea it would fix my performance issues, though it does make sense that as the battery wears out its power output would lessen I presume it is a similar situation this time. As the battery runs down its power output dips slightly.