Usually they get tired of the struggle and stop reading. The retention of the information usually doesn't happen unless they go back and reread the section. Going through school with SSS is very difficult. Just think what it would be like if you never did well on a test no matter how much you studied. It takes a strong drive and a persistent personality to stick with it and not drop out.
Spelling has a lot to do with reading.
Usually good spellers read a lot. If you hate to read then spelling won't come as easy. People with SSS end up memorizing the spelling of words or sounding them out. This might be OK if the English language didn't have so many exceptions to the rules. People with SSS may have a depth perception problem making driving a little difficult especially at night.