Macbook Runs super slow after i remove power cord

I am having this MacBook Pro (13-inch, Early 2011). It has 16GB RAM and is running on a Samsung 250GB SSD drive.

When i am running my system with power cord connected, its super fast. But when i remove the power cord and try to run off of battery, my system slows down considerably. Almost becomes unusable.

The fans run at normal speeds and there is no visible change. I did a fresh install of El-Capitan, as some suggested on the forums, but even that didn't help. My current battery cycle count is 310. What could be wrong with my system?