Chosen Solution
I noticed my Mac had a swollen battery and I took it out and bought a replacement on ebay. I replaced it this past Saturday and it was lagging so bad that I though hmm maybe my memory is too full so I wiped it. That didn’t make a difference I ran a disk repair nothing as well help….
Try an SMC reset and NVRAM reset (links below). The 2012 and older MacBook Airs will lag when the battery is very low, so this could indicate an issue with the new battery. As long as the fan is not spinning full speed (indicating a sensor issue) it will either be due to low battery or a bad battery. https://support.apple.com/en-ca/HT201295 https://support.apple.com/en-us/HT204063
