I believe Rhovanion and the lands east of Rhûn were to near for a true “Asian” population. Tolkien had in mind something like a “proto-Europe” and so it is likely that the Easterlings we heard from were akin to the old sytian peoples. We even have something of a parallel between history and Middle Earth “history on this particular subject:
It exists a certain level of parallelism between the Ostrogothic kingdom in Ukraine and the Kingdom of Vidugavia and his descendents in Rhovanion (and the fact that Tolkien used gothic names to “translate” proto Rohirim names is certainly not a coincidence neither). Considering this, it becomes plausible that the nomads living nearby the Northmen were akin (at least culturally) to the sytian peoples that lived near the Ostrogoths in our world. Further, the few descriptions of the Wainriders remind me much of the Cimmerians too, adding to the idea that they were similar to the Iranian nomads of our world.
We come, we are not seen, and inevitably, we conquer.
First and Last, there is Duty.
Romulan Proverbs