Gearlog discovered Apple notebooks run a lot slower if you're using them without a battery installed. Some users remove the battery when connected to AC power to extend the life of their battery but while upgrading his notebook, the Gearlog reporter found out that this results in a massive performance drop due to a weird safety measure from Apple:
After some digging, I found that the performance drop is documented on Apple's support Web site--though honestly, Apple's reason for forcing a drop in processor speed doesn't make a whole lot of sense. Apple justifies the speed reduction by explaining that it "prevents the computer from shutting down if it demands more power than the A/C adapter alone can provide." But what happens when a battery is no longer able to hold a charge? Does the processor speed drop in that case as well?
We benchmarked our 2.53-Ghz MacBook Pro with 4GB of RAM using Cinebench R10's multiprocessor test, and achieved a score of 5,549 with the battery present and 3,504 with the battery removed.
Lesson learned: The battery stays in. If you're a MacBook or MacBook Pro owner, you'll get the best performance out of your system when you leave the battery in. A small extension of your battery's life span is not worth a 37 percent drop in performance.