MacBook Air running slow, CPU High, Loud Fan when idle

Please read If kernel_task is using a large percentage of your Mac CPU - Apple Support.

Review the links within it, and if nothing appears to stand out then an SMC Reset is justified: Reset the SMC of your Mac - Apple Support.

If you are unable to resolve the problem please write back for additional suggestions.

A runaway kernel_task can be due to a number of factors, such as:

• a failed or disconnected temperature sensor
• cooling system inefficiency (an inoperative exhaust fan on Macs so equipped, possibly clogged with dust, including the heat sink)
• a worn out, missing, or non-Apple (user-installed) battery on portable Macs
• similar hardware-specific causes including internal hardware additions or modifications, external USB and other connected devices, or the driver software required to use them.

If you can find no other explanation for its behavior, the SMC Reset is justified, but you did that to no effect.

Consider running Apple Diagnostics: Diagnose problems on Mac - Apple Support, although only two outcomes are possible. It will either report "no trouble found" when trouble clearly exists, or it will identify a specific fault that only Apple would be able to fix. So you will need to contact them anyway, but certainly let them know the results of Apple Diagnostics.

That Mac is suffering from a hardware fault. Contact Apple to have it evaluated: Official Apple Support