To expand on the previous answer.
t2 class instances use a burstable CPU model. Instead of having access to full CPU resources all the time, you get access to full CPU resources base on a CPU credit model.
When your instance is idling, the instance is building credits up to a cap. The rate and cap vary on instance size.
When you use CPU resources, you spend those credits. Once you run out, your CPU usage is capped very low until you build up more credits.
