I could only see someone calling the HD600 "slow" or "not hi-fi" if they're coming from something like an HD800 or SR009. Those descriptors are best applied in a relative sense of course
But even when going back and forth between my HD800/HD600 it's not what I would call a massacre. Sure the HD800 wins in most technical aspects. You could call it more "hi-fi". The HD600 has a certain bloominess/fatness/muddiness in the bass that is readily apparent when you compare it to something cleaner in the bass. And yes soundstage isn't as wide/deep, imaging isn't as realistic. But the HD600 holds its own. You can compare the HD600 to more than a few "high-end" headphones and it'll utterly destroy them.
There's a reason these babies have been on the market for 2 decades and haven't really lost any of its value; it's just that good