There are few laptops that can match the power of a desktop. Desktop CPU's have a larger cache on board allowing them to pre-fetch instructions to speed up processing. An AMD 2500 XP-M has a memory bus speed of 266 Mhz where as it's desktop equivalent 2500 XP Barton runs 333 Mhz. %25 faster. Desktop hard drives are running 7200 to 10,000 RPM with better air flow which enhances performance. If you were to RAID two hard drives(something that would be very hard to do with a laptop) you gain even more speed. All of these things combine to make a much quicker system. Heat can be another issue for laptops. The hard you work them, the hotter they get, the more the performance degrades. Is portability a big deal for you?
