We currently working on level streaming performance, it’s in our high priority list. You will see improvements in next 2 releases.
Editor does not stream sub-levels from disk when you run PIE, it duplicates them in memory. That is probably why you see shorter hitches in the editor.
Right now streaming does not work well on sub-levels with high number of primitives. By default 4x4km landscape tiles use about 1000 components, try to create landscape tiles with smaller number of components, it should help.