Question: How do you monitor format speed?

Caus you can rearrange a DVD structure and recode the VOBs with special subtitles, etc.

You could try another optical drive? Maybe another brand. Could be the laser it's using and the disks may have faded, and another drive might work.

Use Badblocks and write to the entire drive to check for bad sectors.

I bought a cheap HBA because then I can virtualise the PCI-e card to a VM and use it in ZFS.

Granted I could probably do the same with an onboard SATA controller, but I have more faith in a dedicated controller for my array.