MacBook Air 2012 very slow even in recovery mode

Resetting SMC as well as NVRAM could solve the issue, though I guess somewhere 's thermal sensor is something wrong.

https://support.apple.com/en-us/HT201295

https://support.apple.com/en-us/HT204063

If they doesn't work, you'd better make Genius Bar reservation or take your MacBook Air to Apple Authorized Service Provider to be examined it.

https://www.apple.com/retail/geniusbar/

https://locate.apple.com

HDD(??) temperature sensor and fan speed are something wrong.

You'd better make Genius Bar reservation or take your MacBook Air to Apple Authorized Service Provider to be examined.

Also should replace defective battery.

The visible problem is an absolutely worthless app. Lead melts at 621.5 degrees F. If your hard drive was really generating 1570 degrees of heat, your Mac and home would burst into flames. Wood spontaneously catches fire at 700 degrees.

Knight52 wrote:

I already have the defective battery inside

?

A Mac won't work with a defective battery.

Knight52 wrote:

So you are saying the slow is due to a defective battery?