I also was just noticing that the OP is talking about a child with processing speed in the 80th percentile or above not a scaled score in the 80s. Although that is considerably lower than scores in the upper 90s on the other indices, it isn't so much lower that I'd be thinking LD. Many of us with kiddos with concerning discrepancies have kids with differences of 50 percentiles or more.

Study Strategies for Accelerated Learners