Check your encoding settings! Also if you use an iOS client, I highly recommend Swiftfin, as it seemed to support direct play on some files the Jellyfin app wouldn’t play at first. I’m still new to it too, but after I got my GTX 1080Ti set up on the right encoding settings, it’s been nothing but butter with everything I throw at it.
I though Nord has always struggled with security. They had a breach a couple years ago right? Customer info got leaked?