T2 instance types provide burstable CPU capacity (atop a fixed baseline) governed by a number of CPU credits which are accrued during time the instance is operating within the baseline CPU allocation. One CPU credit offers one minute of full CPU utilisation.
Monitoring of CPU credits is important to ensure that your workload's performance (or your wallet, if unlimited) isn't being harmed by overrunning the available CPU credits. The following CloudWatch metics are exposed:
CPUSurplusCreditsChargeddetails the number of surplus CPU credits the customer has been charged for consuming over the baseline and burst allowance.
CPUSurplusCreditBalanceprovides the number of currently accrued and available CPU credits.
CPUCreditUsageprovides the number of credits burned over the measurement period.
CPUCreditBalancetracks the number of available and unspent ("accrued") CPU credits.
Unlimited instances can burst beyond limits for an extra cost. It's likely more cost-effective to upgrade than to consistently run beyond the credit allowance.