Load Stream Level causes rendering to freeze

While I tested my launched game (on PS4) with development build, the rendering occasionally got frozen when I did Load Stream Level.

According to the name of this node, the loading is async and should not block even a little bit of rendering.
As I traced the source code, the async loading is handled by a function named StaticTick, called by GameEngine::Tick, and StaticTick further called FAsyncLoadingThread::Get().TickAsyncLoading, so it does look like an async load.

The parameters in Project Settings/Engine/Streaming/Level Streaming are all default value.

So the question is simple: Are these freezing normal? Should Load Stream Level behave perfectly that causes not any rendering freeze/frame rate drop?