This should not be the case. I have open an open world map with a lot of trees and grass on it, and some other objects. In this scene for example i have 85 fps, 1920x1080 res, dynamic lighting:
And this is sunset position where shadow count is a lot higher because shadows are much longer. I have a decent komp; i7 2600k, 16gb ram, gtx 1070. So my entire komp costs around 700usd.
Sure, back then in 2016/17 UE4 was slow, but looks like things have changed. Although shadows performance are still an issue, i mean shadows should not be so demanding.