Async loading child with large dataset doesn't always work, it doesn't works well with nodes that were rendered at the 1st load.

Steps to produce:
_ Create tree with virtual scroll and large dataset (10k) => tree is rendered fast and good when page load.
_ Create getChildren option which return promise with 10k nodes.
_ Click on nodes that were rendered at the 1st load => observe the delay time (~12 secs)
_ Scroll down and click on nodes that are down below => node is expanded quickly without delay.