From a no longer existing Apple support document...
Apple essentially down-clocks the CPU to prevent the system from shutting down if it happens to demand greater power than the AC adapter alone can provide:
"If the battery is removed, the computer will automatically reduce the processor speed. This prevents the computer from shutting down if it demands more power than the A/C adapter alone can provide."
If you google it you will find plenty of reports as far back as 2008. It also happens with my MBP from 2009 which I ran for a long time without battery after it blew up. Now, after installing a new battery, it runs noticeably quicker.