We have already logged this improvement as a feature. You can track the status of this feature by using the following feedback report link.
Feedback: https://www.syncfusion.com/feedback/34761/provides-the-virtualization-support-to-the-listbox
As we have already lined up some major features, we could not start this feature immediately. We will implement this support and include it in any of our upcoming releases. We appreciate your patience until then. We will update the status of the feedback link to "scheduled" once it is taken for implementation.
