Intro to Databases Week 5

If indexes are supposed to speed up performance of query, what does the author mean by a slow index?

I believe that by saying a "slow index" Markus Winand is saying that just because your table(s) have indexes doesn't meant that the lookup time can't be slow.  That there are a couple of reasons you might have a slower than expected lookup time when using indexes.

One reason might be that depending on the data stored in the database when querying your query might have to perform multiple leaf node reads.

Second, that data stored in the table is often spread across different table blocks, i.e., the data you're looking for might not be in two rows near each other.  And so depending on the backing store, that can add additional unforeseen time.