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Pre-processing uses a lot less memory and takes a lot less time than post-processing
From KB Article
Using ExcelTemplate alongside ExcelApplication
As discussed in the introduction, ExcelApplication was designed for the purpose of creating more dynamic content, but as a consequence it is more memory intensive. ExcelTemplate, on the other hand, was designed for the purpose of importing large amounts of data, and much less memory is required per cell for an ExcelTemplate object. If you are experiencing memory problems with ExcelApplication because you have a large amount of data, the best option is often to use ExcelApplication to create a template, and then use the ExcelTemplate object to actually import the data.
For example, you can create a Workbook with ExcelApplication and insert the formatting, charts, or dynamic content that you want to appear in the final workbook, along with data markers where you want the data to appear. You can then open this Workbook with ExcelTemplate to populate the data.