My eldest DD's PSI declined from 7.5yrs to 10yrs. On paper. I think at 7 she just did the test as fast as she could and didn't care about neatness etc. Or possibly even accuracy. At 10 she had perfectionism and anxiety in the mix too. She had OT assessments at a similar times as well, and her handwriting improved on all measures (against age peers both times). Which is to say that at 7.5yrs her handwriting was below average in all measures and by 10yrs it was above average for neatness and marginally above average for speed. She's still not fast, but she's not that bad and her writing is very legible and fairly appropriately sized. I personally don't think her actual processing speed is any worse than it was, it might be better, but given that she has ADHD-I I am not going to complain about a low score qualifying her for extra time.
I think that the WISC PSI subtest are very easily skewed by personality and can pick up such a wide range of issues that it is really hard to pinpoint whether it really means anything real, and if so what. Is it anxiety? If so is the child usually anxious or only about this test? Or most tests, but not school work? Is there a vision problem, a co-ordination problem, an attention problem, an actual physical problem with handwriting? You don't get any of those answers from the WISC, only a hint that you need to investigate something.
My 2nd DD is actually quite co-ordinated, has good sensory integration and is reasonably attentive, but physically cannot hold a pencil well without a custom brace and special pencil grips, this might be why her PSI score was 50th percetile, but she also just plain refused to finish the test and got a perfectly average score in half the available time, which the tester would not have mentioned on the report had I not brought it up. The tester seemed to assume that this was all DD could figure out - hello, she did each item perfectly twice, she feels she has proved mastery, she's not unable to figure out what to do, you didn't emphasise that it's a race, or that she has to do as much as she possibly can, you only told her what goes where, she's shown you that she understood the instructions, it's a really boring test and her hand hurts so she's done.... She's also not a fast kid, but I don't think the PSI score she has tells us anything real at this point in time...
The child you are describing here though is a child that sounds like they need a full neurospsych evaluation to me.
Also - I could be wrong but I believe that by your child's age the PRI subtests are timed - which means that if whatever caused the lower PSI was also at work during the PRI tests then their actual visual spatial reasoning ability is quite likely underestimated by that 135.
