Ridiculously slow MacBook Pro 15'' 2011 with low battery and El Capitan
Perform Apple Hardware Test to check for any hardware failures. To do so, power up your Mac, then immediately press and hold the D key. Keep holding the key until you see the Apple Hardware Test icon. Thereafter follow the steps as mentioned in the support article, How to use Apple Hardware Test on your Mac.
Reset SMC and NVRAM.
If the hardware test passes, try reinstalling OS X El Capitan. If the problem persists, fallback to Mac OS X Lion.