Why is my computer slow?
Why my macbook air (13-inch, Early 2015) is slow?
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Many possibilities! Lets start off with the basics what is your SSD drive and how large is it and how much free space does it have.
Next did you try running the onboard diagnostics? Restart your system and press the D key. Depending on your macOS this might not work - Thanks Apple for breaking it! Let us know what you get for an error if it works.
Reference: Mac startup key combinations
by Dan