Having seen his Heps races (indoors and out), I think "physical AND mental" might sum it up.
In his bad races (outdoor Heps), he looked like a guy whose feet hurt. But that may be an artifact of "sorry for myself" running, which can actually affect someone's mechanics.
I was going to say that his up-and-down-ness is characteristic of some other runners I've known (and coached), who suffered from anemia. But then it's hard to explain his performance in last fall's Heps xc race, wherein he was "well up" for much of the race and then utterly slacked off--runners with anemia problems on a given day generally look bad almost from Step One.
Anyway, the fact is that if you're not having a great race (or simply have a bad patch), it's easy to alter your mechanics unconsciously and rapidly make yourself even tireder. In line with this, over the years I've noticed that taller distance runners (Korolev certainly qualifies) seem more frequently to fall victim to this. (By contrast, smaller guys generally look great right until they crack.)
I wish him well, and hope he and his coaches get things sorted out.