Laptops generally have pre-installed power management software that can throttle performance to ensure good battery life. If you haven't changed any settings, the default will usually reduce performance any time it's on purely battery power, regardless of charge. The assumption being that, as a laptop, it's more useful to have 6 hours of operation at 50% performance than 2-3 hours at 100% performance. It can always take more time to get something done, but once the battery is low, you can't do anything.

However, if you're connected to an external power source, the laptop will generally assume that power is plentiful, so it will give you 100% performance as long as it can also charge the battery as it does so.