I'm assuming your script first orders youtube to display all favorites then searches amongst them and removes everything except from the one that matches.The whole process only took 1 sec for my 400 favorites and yep it works just great.
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That's spot on. There is no other way I can see to implement this feature. But usually if you can load the videos quick enough the script only takes around 10 milliseconds to run.
Thanks for the great review.