It's no accident Rijndael was chosen over Serpent for AES as it left the door open for possible future NSA exploits. Muh speed was given as the excuse, ignoring ever increasing computing speed. The chosen algorithm would also receive a hardware instruction set on most architectures.
https://web.archive.org/web/20231124173420/https://csrc.nist.rip/encryption/aes/round2/conf3/AES3FeedbackForm-summary.pdf