MS Office Interop is slow and even Microsoft does not recommend Interop usage on server side and cannot be use to import large Excel files. For more details see why not to use OLE Automation from Microsoft point of view.
Instead, you can use any Excel library, like EasyXLS for example. This is a code sample that shows how to read the Excel file:
ExcelDocument workbook = new ExcelDocument();
DataSet ds = workbook.easy_ReadXLSActiveSheet_AsDataSet("excel.xls");
DataTable dataTable = ds.Tables[0];
If your Excel file has multiple sheets or for importing only ranges of cells (for better performances) take a look to more code samples on how to import Excel to DataTable in C# using EasyXLS.