Database (Excel) Access Speed - Using Open XML SDK in Visual Studio C# (DOM Approach)

I mostly write number-crunching programs using Visual Studio C# (2019) where I am simply taking input data, calculating results and displaying it. No complicated Network or Internet programming. Think first or second college level programming coarse from the early 1990's.

For inputs I was reading in data from an excel file using the following directive:

using Excel = Microsoft.Office.Interop.Excel;

This proved to be very slow when executing the program. I then learned this way of accessing an Excel file is no longer supported and has been superseded by Open XML SDK. Please see the following link to the Microsoft Dev Center page:

For what I want to do the Document Object Model(DOM) approach seems most appropriate for the thousands of individual excel cells I want to read as input data. However, the Microsoft Dev Center is certainly not the most user-friendly resource and the code example provided for reading an Excel file using this DOM approach is writing to a console which I'm not using. I never did get my code to work.

Long and short of it is, I got my code working using the GetCellValue Method:

However, this 'GetCellValue' method is still taking way too long. I need to read in thousands or tens of thousands of Excel input data cells in seconds or fractions of seconds not 20 seconds to a minute.

I think if I had an example of the DOM method reading in Excel data to an Array Variable (instead of writing to the console) it would help. Can anyone provide an example of such code?