I have encounter the same problem two days ago, but finally figure out. The Truth is, Epic implement level streaming function very well, the problem is we don’t know how it works and how to use it.
[Load Streaming level] node will load the level data from disk, so it is slow, time consumes depend on your streaming level size. (Mainly happens in game mode, seems it will cache in editor mode).
The Best way is You can load the streaming data at game loading time, just tick the Initially Loaded option at Level Details panel, or use the LoadStreamingLevel node. then at runtime, use [Get Streaming Level] node, feed in StreamingLevel name or use the Unique Instance Name created by [Create Instance] node and set the variable Should be Visible, you will not have any stutters.
My situtaion was I have two levels in one map, one is Day exterior scene with more than 10000k poly counts scanned data, and one is night interior scene with high quality lightmap baked, I want to transition the two map seamlessly, use the method told above, the streaming works perfectly fine.