Have you tried the same thing with the same discs in another computer? I have some CDs that import quickly and some slowly too, and I think that it is due to the dynamic balance of the CD -- that is, how true the plane of the disc is. If a disc is slightly warped or wavy or has a sticker on one side or something like that, even if it is not perceptible to the eye, it can generate a lot of vibration and so on when it is spinning at high speed.... I think you should find that the same CD consistently rips quickly or slowly, although a CD that rips at <1x is pretty sketchy....
Also, occasionally (two CDs out of about 250), I have had CDs that either rip funny (with pops and clicks and hiss) or will not rip. I don't know what causes that. At the time, when I had just bought my iBook, I actually ripped the CDs on my desktop PC.