SFP+ 10Gb Optical/Fiber/AOC very slow, but DAC fast

I have a strange issue.

• Identical Mellanox cards (purchased in same lot, NEW)
• Identical Mellanox transceivers (same lot, but USED)
• 30m (98 ft) OM3 optical cable (NEW)

The speed I get is around 25 Mb/s, which is even slower than HDD read speed.

I switch to 3m DAC cable and the speed goes up to like 4 Gb/s. I switch back to obtical and it's super slow again.

What could the issue be? Do optical transceivers degrade over time? (I bought them used, and they could have been used 24-7 for 10 years straight.)

Could it be the cable? I have most of it bundled in 1 ft diameter coil, and have always been careful not to bend the fiber anywhere too much.

What troubleshooting steps should I take next?