A colleague gave me a 2010 MacBook Pro and I notice that it runs very slow when running on battery. Boot-up takes a very long time and everything is lagging. It is practically unusable. When the power supply is plugged in however, it works normally. I tried the usual prescribed troubleshooting from Apple to reset the SMC but that still didn’t fix the issue.
Finally I came across this discussion on Apple Support Communities. The root cause seems to be faulty temperature sensor damaged by liquid spill. Because the sensor is faulty, macOS assumes the system is overheating and drops the CPU cycle which in turns makes everything runs much slower. Read More