Usually, a large list of values means a lot of nodes to calculate before displaying the Widget, which could mean a performance degradation.
Instead, you can optionally let the Tree View only load the lists of values on demand : until the user unfolds a node, the children values are not loaded.
If the node unfolded has many values, this may mean that it will take some time to unfold.
On the other hand, the initial loading of the Widget is much faster, and most nodes will open fast if they each have few values inside their immediate children.
To enable this behavior, check Enable Lazy Loading :