Thanks for contacting Syncfusion support.
We have analyzed your query. In your sample, you set the ColumnSizer as Auto which leads to slow loading. When ColumnSizer is “Auto”, column width will be calculated based display values in column which will affect the loading time of SfDataGrid. So we recommend you to set individual width to columns based on its content or set ColumnSizer value other than Auto , AutoWithLastColumnFill, AutoLastColumnFill or SizeToCell.
Please let us know if you have any further assistance.