Rohan is a lot more demanding on your PC that some of the earlier regions like Endewaith or Great River. Some people have to lower their visual quality via the graphic setting to get better performance.
I have not had to lower my settings but the game does not always run smoothly in Wildermore or East and West Rohan. I will hesitation (the game will freeze for maybe 1/3 to a full second). I was noticing the past few days it takes a long time for my horse to appear on occasion. I will be sitting on an invisible horse for a few seconds. Stable horses are the most funny. I will mount up. Off I go down the road on an invisible horse. Finally the stable horse appears.
This situation for me is not consistent. It is sporadic issue. It only happens in the new regions Riders of Rohan or later.
Unless stated otherwise, all content in this post is My Personal Opinion.