By default the RadSpreadProcessing creates workbooks that have 1048576 rows and 16384 columns and there is no way it can import the entire CSV file. If you split the file in smaller CSV files the RadSpreadProcessing will be able to import them but it might not be able to handle that large amount of data fast enough.
If you do not need any specific functionality that RadSpreadPorcessing provides I suggest you parse the CSV file with you own CSV parser implementation. It would be faster. Here you can find more information on Parsing CSV files in c#
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