S3A now has a default list of credential providers to pick up AWS credentials from
The environment variable provider added in
HADOOP-12807 should go before the InstanceProfileCredentialsProvider one in the list, as it does a simple env var checkup. In contrast InstanceProfileCredentialsProvider does an HTTP request even when not running on EC2. It may block for up to 2s to await a timeout, and network problems could take longer.
Checking env vars is a low cost operation that shouldn't have to wait for a network timeout before being picked up.