Processor speed is really no longer a useful factor when choosing a computer, particularly a laptop computer. As processors have gotten faster, other bottlenecks have become apparent. Memory and disk bandwidth are now more important specs than processor speed.
You'll find a computer with a solid-state disk to feel much faster than one with a hard drive, in every way. Programs and files will open more quickly, the computer will boot, wake and sleep faster. Your entire experience with the computer will be greatly improved.
Next up, consider portability. Look for a laptop that is lightweight, thin, and lasts a long time on a charge. You'll be carrying it around everywhere, you'll be out of your dorm room for long periods of time, and you'll find yourself in lots of buildings where many people are competing for a power plug. You'll really enjoy a laptop with great battery life.
I realize these are very utilitarian features -- thin, light, good battery life, and with a fast disk -- but utilitarian features and creature comforts are what will ultimately make you love or hate your laptop.
You'll run a few compute-intensive programs during your undergraduate education, but you'll probably never run anything that takes more than a minute or two to complete. Don't sweat it.
- Warren