It affects 13.3-inch polycarbonate (“white”) MacBooks released earlier last month. Some of those had the following issue:
…may display slower performance when started up using battery power only. Restarting using battery power may not clear the condition.
Main site: http://support.apple.com/kb/HT3435
Download: Support.Apple.com Downloads
All EFI/SMC Firmware Updates: http://support.apple.com/kb/HT1237
(check the above link for eligibility)