[anonymouse008]

Check your kernel_task in Activity Monitor... if it's hogging all the processing % it most likely means the processor is being throttled.

The throttling sometimes happens because of a bad temp sensor / connection. Sometimes it's the charging port as well

[bayindirh]

Or during long re-indexing sessions which happen after OS updates generally.

[jimmaswell]

I've had laptops that got slower over time that turned out to be full of dust inside. Easy fix with an air hose or anything to the same effect.