Whole-word recognition: Mature readers rarely decode. They generally read larger chunks at a glance--a whole word or even a whole phrase at a time. This is why mature readers can read fast. Reading skill acquisition should move from decoding to whole-word recognition fairly rapidly.
Being a slow reader generally slows down comprehension, and makes reading effortful, thereby reducing motivation to read. This creates a vicious circle: The slow reader doesn't like to read, so doesn't read much; not reading much keeps the reading speed slow, making comprehension effortful, so the slow reader doesn't like to read ... and so it goes.