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? 

Honestly, I didn't quite understand the post on this website but, I do know that indexes are abused by a lot of developers. From my experience, developers will see that a query is taking a while and jump to the conclusion that it can be sped up by using indexes.

Then they throw indexes on just about everything.

But, let's say you're managing a database with a ton of CRUD activity then those indexes start to slow things down as the database is constantly trying to re-index things even as they are changing.

that's just one example but, there are several scenarios where indexes tend to be hurtful rather than helpful. And that's not including the added complexity of implementing indexes.

IMHO devs need to go the extra step and query their data sensibly. Break up large queries with pagination and filters. Or if you realllllyyy need to query a large dataset you need to expect that it will take a little while, slap a loader on there to keep your user entertained or something. idk.