The speed can depend on a number factors.
Whether you rip from the DVD or use DVD Decrypter and do it from the HDD.
The amount of compression. Generally the more compression, the longer it takes. No compression and you can get some really fats speeds.
If you use AEC, this can slow down Shink considerably. Max Smooth is the slowest.
The number of audio and subtitles being removed will slow the speed down.
On a Athlon XP 2000. I get around 25,000 KB/s with no compression. With average compression and no AEC I get around 7000 KB/s. Using AEC the speed can be around 900KB/s for Max Smooth to 2,5000-3000KB/s for default AEC.
The only way to increase Shrink's preformance is a faster processor. 512MB of RAM does help but anything more is not necessary.