DD had a 60 point gap between her PRI and PSI scores on the WISC (PSI being lower), yet she is a fast and fluent reader. I worked with kids struggling with reading, and I could see some of them had processing issues, as they read very slowly but accurately. One boy read the same slow speed no matter what reading level I gave him. And he never seemed to speed up no matter how many times we read the same paragraph over and over again. I'm not sure how he would have done on other processing tasks (like coding on the WISC) but I think processing speed can vary wildly depending on the specific type of task.