Don’t bother using your MacBook or MacBook Pro without the battery installed unless you’re cool with a huge performance decrease. Gearlog notes that, in the process of doing the thankless task of RAM benchmarks, the MacBook’s performance, as rated using Cinebench, dropped some 37 percent without an installed battery. That is, when plugged into a power outlet without the battery installed—some people do this to extend the life of their battery, having it installed only when necessary—the MacBook suffers quite a bit.
Apple saysthis is a feature, not a bug. The idea is that the MacBook can detect wether or not there’s a battery installed. If not, performance drops in order to “prevent[s] the computer from shutting down if it demands more power than the A/C adapter alone can provide.”
So, unless you’re a glutton for punishment, keep the battery in there.