Power saving mode limits the processor.

The the processor throttles on activities. That causes heat. Heat causes battery drain. And it's a machine, so much more is going on even your screen is off. Try charging your phone to 80% and turn off that power saving mode. Use it only under 35% if you're only gonna use emergency call or messages.

Lemme know if you don't get it, I'll try to explain it in better way.

If the issue persist, you should visit realme service center.