MBP Running slow when its on batteries .. working fine when plugged to the charger ?!

Hi there !
I noticed that my MBP is running very slow when it's on batteries.. but when plugged to charger it works fine..

Is the battery drained already ? I purchased it OCT 2011 but has been using it lightly for almost 5 months ..

P.s : I cleaned it up coz I thought its a space or piled files issues but it didn't work

Thank u in advance 🙂