The instances in the t2 and the t3 instance family, i.e. the instance types that start with either t2. or t3., are burstable ones. That means the instance collects CPU credits over time that can be used later. If you use less than what you get in the long run – the baseline performance, between 5% and 40% of the available CPUs – you won’t even notice how this system works. But if you use more than that, the instance will either get throttled or you’ll get charged for the excess usage.