I think being 5.5 in full-day school is almost certainly a contributing factor. The "slow motion movie" thing (nice analogy!

) is also a good bet. More challenge generally cures the latter but not the former. Giving more challenging work is probably a good way to find out what the problem is, actually.
As for the observation of the school psych: honestly, I think I tend to doubt what school professionals say whenever they're in the position to be addressing large numbers of parents about general problems kids may be having. Anything they say is necessarily geared to the "average kid" (whatever that is...) and is not necessarily relevant to any one specific kid. Especially not an HG+ kid! It just means to me that I should take with a grain of salt anything they say. I don't mean to be unfair; I'm just trying to be realistic. We're not really their usual audience!