They are slower for both data access and retrieval, as well as write operations, leading to longer wait times when saving, updating or retrieving data.
When you detach an EBS volume from an EC2 instance, the data on it stays safe and can still be accessed, as long as the volume is not deleted. You can attach the volume to the same or another instance later to get the data back.
Attaching additional EBS volumes to an EC2 instance gives you more storage and helps keep the operating system separate from your data. This makes it easier to manage, back up, and improve performance.