This is a debatable topic. We wrote a blog post about this as well: https://supertokens.io/blog/are-you-using-jwts-for-user-sess...
Presumably it's just using a HMAC, I think if it used RSA or an asymmetric signing scheme, the performance would look much different.
[0] https://www.javamex.com/tutorials/cryptography/rsa_key_lengt...