https://www.bleepingcomputer.com/news/security/ten-notorious-ransomware-strains-put-to-the-encryption-speed-test/

Researchers have conducted a technical experiment, testing ten ransomware variants to determine how fast they encrypt files and evaluate how feasible it would be to timely respond to their attacks.

Ransomware is malware that enumerates the files and directories on a compromised machine, selects valid encryption targets, and then encrypts the data, so it is unavailable without a corresponding decryption key.