[blt]

I would go for a comment like "a simpler naive implementation is in commit A0348C. It ran approx. 2.5x slower in 2019."

[jlokier]

I would do similar but include the commit timestamp and/or words from the subject to help locate the code if the commit id can't be retrieved.

Rebasing and other means of moving patches (email etc, adding signed-off-by) can cause the original commit id to become invalid, and eventually unretrievable.