Thank you for contacting Syncfusion support.
We are unable to reproduce the reported exception from our side. So we have prepared a sample as per your requirement which can be downloaded from the following link.
Kindly share us the modified issue reproducing sample along with your machine configuration which will be helpful for us to provide you a prompt solution at the earliest. In XlsIO, There are few ways to improve the performance and memory optimization which is explained briefly in the following UG documentation link.
UG documentation link: http://help.syncfusion.com/file-formats/xlsio/improving-performance
Please let us know if you need any concerns.