There's no reason it would be against the TOS, but there's no particular reason it'd make a good backup either. Youtube doesn't limit its weird, arcane, random-ass mutes and takedowns to public vids. At the very least make sure it's not your only backup.
Not really. Depending on how it works, it can slow down the browser itself due to needing to inspect and change content. Simple URL filters will, if anything, speed it up by blocking you from even trying to download unnecessary or malicious stuff.
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Multiple Adblockers in a row - does it make sense or is it even harmful?
Hi, I was wondering, is it useful to use multiple adblockers in a row?...