Cellular usage is very power hungry. 40minutes on Cellular drained to 75% is not too bad. You got to count other factors like multiple applications opened at the same time, a web site that have FLASH contents like Youtube..etc. And CPU cycle of your device, if your CPU is high then your device will be low on power faster as well. Also consider reflashing your device and eMMC so you get a fresh start if you bought it Secondhand.

Let's hope it fix some of your problems.