MacBook Air wasn't used in a long time, not charging, very slow

My sister bought a MacBook Air 2018 when it came out but she barely used it because she still had her laptop given to her by her office. She used it occasionally but barely anything, she’s certain that she hasn’t used it for more than 4 times. The laptop was stored for some months in its original box before she wanted to use it yesterday. I don’t know how much battery it had before it was stored but probably not empty.

Now the laptop is literally unusable because of how awfully slow it is. Even doing something as simple as checking “About this Mac” takes a very long time.

I already tried to reset the SMC and NVRAM several times but I’m not sure if it actually did reset both those things because there isn’t a confirmation flash or anything like there used to be with the magsafe, where the flash would turn off and then on again.

The laptop only works with the charger plugged in, the charge level shows as 0% and it displays a message along the lines of “Replace now”. The fan is also extremely loud all the time.

The laptop has never been disassembled before.

The only thing that occurs to me is that the battery voltage dropped too low in storage and now the laptop won’t detect it as a good battery. She’s already tried leaving it to charge overnight to no avail.

Currently, I’m in the process of trying a clean Mac OS install but I doubt that will work because I’ve been waiting for the laptop to boot into the install usb for about 30-40 minutes now.

Thanks for reading!

Edit: just checked how many cycles the battery has, it has 5.