Macbook Air suddenly very slow following Big Sur 11.2.3 update, 250% kernel_task

Run the Apple Diagnostics to see if any hardware issues are detected.

Try booting into Safe Mode to see if things are any better since Safe Mode disables third party drivers, Startup Items, Login items and even some Apple drivers like the built-in camera. If Safe Mode works, then you have a software issue so make sure all your third party apps and drivers are completely up to date.

You should also run Disk Utility First Aid on the hidden Container. Unfortunately Disk Utility now hides the physical drive and Container from view by default since macOS 10.13 so you need to click "View" within Disk Utility and select "Show All Devices" so that the hidden Container appears on the left pane of Disk Utility. Even if First Aid shows everything as "Ok" click "Show Details" and scroll through the report looking for any unfixed errors. If you find any unfixed errors listed, then you need to erase the whole physical drive before restoring from a backup or clone.

An SMC error is unlikely to be caused by the battery although I cannot rule it out completely. More than likely the SMC error is due to an issue with the SMC chip on the Logic Board or the Logic Board itself (or possibly a bad or loose I/O Flex Cable or a bad I/O Board if the SMC isn't receiving the proper power from those components). Of course Apple doesn't replace a bad SMC chip, but instead will replace the whole Logic Board. You should have Apple or an Apple Authorized Service Provider examine the laptop to provide you with an estimate for repair.

According to this Apple article your laptop is still considered "Vintage" by Apple so it can still be repaired as long as the necessary part(s) are still available. The only part which may not still be available is the Logic Board since all the other parts of this laptop are shared by later models which are still fully supported by Apple so those parts will still be available.

https://support.apple.com/en-us/HT201624

zneyaz wrote:

Ran diagnostics and it showed errors PFM007 There may be an issue with SMC and PPP003 (can’t remember the exact code) There may be an issue with the power adapter.

Then your Mac has some sort of hardware issue. Any type of power issue can cause a Mac to run extremely slow and the SMC deals with power management. You will need to have your laptop repaired.