I'm new at this...

[crispy]

A safedisc1 or 2 cd contains read errors on the first sectors of the disc.
They are meant to be "unreadable", or with some readers just difficult ro read.
It is theoretically feasible that a cdrom will read more accurately at a lower speed, as you've found here.
At the end of the day, some readers are better than others. To speed things up with sd1 and 2, you should have fast error skip set in CloneCD, or use daemon tools's "fast dump" feature.
Info available on their website.