Looking into why CD ripping is sometimes slow in rhythmbox, I noticed that once a track has been played (which causes rb to set the read speed to 1), ripping would proceed at roughly playback speed.

This appears to be happening because cdparanoiasrc only sets the speed if the read-speed property value is not -1 (and it treats 0 as -1).  The cdparanoia call that sets the speed (cdda_speed_set) works by calling an ioctl or issuing a scsi command to the device, so creating a new cdparanoiasrc instance or closing and reopening the device won't reset it, meaning that the read speed setting for playback still applies.

Is there a reason it doesn't set the speed when the property value is -1?

I'm currently hacking around this by setting read-speed to 65535 (the field in the scsi command is 16 bits) for CD ripping, which seems to be helping.