Processor Speed for PLEX - DS1522+ vs DS1621+

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None of these can transcode 4k on the fly. You can use offline transcoding that will generate an additional file for remote streaming. If all your streaming happens locally at home, make sure your vide files can be played on TVs and other devices. This way you can avoid a need for transcoding. If you do need it, try to find DS1520+. Or if you use Plex, consider Xeon based models or Qnap 73A series with GOU card.

I hope this helps.