Why scanning an index backwards is slower?
The author, Chaithra Gopalareddy,  of the related article, MySQL 8.0 Labs – Descending Indexes in MySQL, explains in a comment why backwards index scans are slightly less efficient than forward scans:

Thanks for showing interest in the new feature. The ~15% cost benefit in forward scans can be attributed to the optimizations done in innodb to favor forward scans over backward scans.
For ex: W.r.t a scan within the page – The records in a page form a singly linked list. To get the next record, a forward scan just follows the link where as the backward scan need to start from the beginning (first slot) till the current slot/record to identify the previous record.
Along with the above, there are some more contributing factors like, during page switch – page latching rules currently defined in innodb favor forward scans over backward scans.

So there are two factors:

the single linked list structure of records inside a page
page latching rules regarding page switches