Huge data under sysContentFolder causes performance problem when uploading media files

Description

Prerequisite:
Must have many (preferably a few thousand at least) folders under content ID 4.

Steps to reproduce:
1. Create a new page in Alloy template.
2. Go to media widget and check the ID of the "For this page" folder (ID=4).
3. Press F12.
4. Choose the network tab then click the upload files by dropping them here link.
5. See that the link below takes a long time to complete:

EPiServer/cms/Stores/contentstructure/?references=4&query=getchildren&allLanguages=true&typeIdentifiers=episerver.core.icontentmedia&dojo.preventCache=1548836611582

This one queries all children items (10,000 items) under ID=4. It takes a long time to finish the loading.