CST363: Introduction to Database, Week 5

The web site "Use the Index Luke" has a page on "slow indexes".   https://use-the-index-luke.com/sql/anatomy/slow-indexesLinks to an external site.. If indexes are supposed to speed up performance of query,  what does the author mean by a slow index? 

Even though indexes are designed to improve query performance, the author explains that an index lookup can still be slow depending on how it is used. The term “slow index” refers to cases where the lookup involves extra work beyond the initial tree traversal. Once the index finds matching values, the database may need to follow additional leaf nodes if there are many rows with the same key and then retrieve each corresponding row from the table, which can require accessing data scattered across multiple disk blocks. These extra steps make the operation slower.