The web site “Use the Index Luke” has a page on “slow indexes”. If indexes are supposed to speed up performance of query, what is meant by a slow index?
An index is fast partially due to the underlying tree data structure, which when optimized properly are generally efficient to traverse when searching. The term “slow index” then refers to an index which is improperly optimized as a tree, and thus not fast in the way that they are meant to be.