When indexing large text collections minimizing the indexing time and the disk storage used to create an index remains important. Indexing optimizations applied to a prototype retrieval system at NIST are discussed in this paper. These include the organization of the index, the use of virtual memory facilities to improve indexing time, an index addressing scheme to decrease index size, and the implementation of term position information extensions using compression. These improvements provided a large decrease in indexing time and moderate decrease in index size for indices without term position extensions. Indices using term position extensions had a more moderate increase in space/time efficiency. 1 Introduction As computers grow exponentially faster, and disk drives become more compact and inexpensive, it seems that efficiency should be less important. However, this is not so, at least in the information retrieval community. If available disk space is growing, so is the amount of t...