From a blown OS, malware to hardware issues (heat, old chips, etc.) that I can't give you one answer.
Are you keeping up the canned air to the vent cleaning over the years? Some folk never do this and I start with a deep clean and usually that's it unless the OS, malware or hardware issues.
Let's hear exactly which Asus this is, age, your maintenance schedule and how you keep the malware at bay.
The screen begins to flicker blue with random squares of visibility but generally distorted
Happens no earlier than 5~ minutes into use and no later than 20~ https://imgur.com/a/184cF

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