Possibly the drive might be having issues reading the disc which results in lowering the read speed. The ASUS BW-16D1HT is a speedy drive. If the slow speed only happens with San Andreas then I'd look more closely at that particular disc.
So far I have ripped four UHDs, and the speed always was about 8,95 MB/sec...
Any way of testing that drive on another system? If it happens with all discs then the slowness is either the drive or something system-specific.
Any ideas from the log?