WD MyCloud slow

I have WD 4TB My Cloud Personal Network Attached Storage - NAS - WDBCTL0040HWT-NESN with 2 more 8tb drives hooked up to it. The other drives have plenty of room on them but the Cloud drive has only 16gb left. All drives have 4500 movies and 36000 TV show episodes !

Which is the minimum gb’s should I have on the main drive so it doesn’t act sluggish ?

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Nothing ?