what the research says

Sound + vision (phonology and orthography)

A great deal of research shows that average reading ability and below-average reading ability are best predicted by the somewhat independent effects of phonologically based decoding and rapid naming (Compton, DeFries, & Olsen, 2001; Cornwall, 192; Kirby, Parrila, & Pfeiffer, 2003; Savage et al., 2005; Strattman & Hodson, 205; Wolf & Bowers, 1999; Wolf & O’Brien, 2001).

Savage, R., Pillay, V., & Melidona, S. (2008). Rapid Serial Naming is a Unique Predictor of Spelling in Children. Identifying and Predicting Classes of Response to Explicit Phonological Spelling Instruction During Independent Composing. Journal of Learning Disabilities 41 (3), pp. 235 -250

The neuroscience suggests that "storing the picture" of a word and "processing the sounds" in it are two somewhat-independent pathways. A student could have a deficit in either or both, making diagnosis more complicated and adding weight to the idea that you have to be "firing on all cylinders" to read and comprehend successfully.

In current models, RAN [Rapid Automatized Naming] is seen as a measure of an underlying structure or timing congruence in the central nervous system that underlies the capacity to accurately and efficiently store detailed orthographic information about words. This structure is seen as being somewhat independent of the structures used for phonological processing (e.g., Wolf, Bowers, & Biddle, 2000).

Savage, R., Pillay, V., & Melidona, S. (2008). Rapid Serial Naming is a Unique Predictor of Spelling in Children. Identifying and Predicting Classes of Response to Explicit Phonological Spelling Instruction During Independent Composing. Journal of Learning Disabilities 41 (3), pp. 235 -250

Focus on skills and skill deficits

Previous research has shown (Francis, Shaywitz, Stuebing, Shaywitz, & Fletcher, 1994), and the results from this study support the notion, that reading difficulties are best characterized by deficits in prerequisite skills that lead to deficits in reading development, rather than by a lag in reading development. Identification of a group of students with persistent deficits over the 3-year period suggests that unless the students acquire the necessary prerequisite skills, they will continue to lag behind.

Boscardin, C.K., Muthén, B, Francis, D.J., Baker, E. (2008). Early Identification of Reading Difficulties Using Heterogeneous Developmental Trajectories. Journal of Educational Psychology. 100 (1) p. 192-208

Skills in this paper was referring to phonemic awareness, word recognition, etc.