Hi Bernard,
Thanks for contacting Syncfusion support.
We have checked the given sample in our machine and it loaded grouped records properly. For your reference, we have attached the GIF image of the grouping action.
Also grouping with virtualization performance will be based on the system memory and other CPU usages. If you still faced this problem please share any information you might find useful for us to analyze at our end.
Regards,
Madhu Sudhanan P