Same experience here and it is not just your cpu, it's the speed of your system. I have one system that has an athlon xp2400 266 fsb, 512mb pc2100 and a cheapo motherboard. I have another system that has an athlon xp 2500 (overclocked to 3150) fsb 333 (overclocked to 400), 512mb pc3200 (with beter latency) and an abit nf7-s and the faster computer can do it 5-10 minutes faster.
Size of the movie matters too. On my fast system, a movie that requires no compression takes not much more than 5 minutes while one that requires heavy compression takes more like 25-30 minutes.
Fyi I am using nero but it is similar to dvd shrink and the same concepts apply.