Well, I tested your project with your template file and it works fine with v7.4.3 (latest version).
I generated the output file with 13MB in size without taking much time on my good x64 bit machine with good RAM. Also, MS Excel does take some time to open the output file as it has a long list of records.
The point is as you have lots of records in your source table (122301) with a certain amount of columns, so the process would demand more memory for sure.
Moreover, in your code, you have specified more number of rows than actual, so Aspose.Cells has to create many extra blank rows which also takes resources, so you may try to change the line of code i.e.
designer.SetDataSource(“LabelPrev”, oReader, 400000);
designer.SetDataSource(“LabelPrev”, oReader, 122301);
Moreover, we recommend you to use DataTable instead of DataReader if it enhances the performance on a normal system with a minimal amount of RAM. For example, you may fill the datatable from the back end and then specify that datatable while setting data source for the markers.
In short, there are measures to be adopted as mentioned above, but if you say, for a huge list of data (as you have), Aspose.Cells would take very minimal/little amount of RAM and resources to process the big process, I am afraid, this is not possible.