Answer
Output Channel's Excel-export path isn't designed to handle large numbers of rows efficiently, and there's no way to configure it to skip auto-opening the file and just prompt a plain file-save dialog instead — it can take hours to complete on genuinely large datasets regardless. The better alternative for exporting a large flat/CSV-style dataset without unnecessary overhead is to write the same query logic as a proper SQL Quick Report instead, and use that Quick Report's own RMB > Export to Excel option. This doesn't offer a 'Save As' prompt either, since it opens Excel directly too, but Excel itself can open even very large files without the same kind of extreme slowdown that Output Channel's own export mechanism runs into, making it a meaningfully better option for large datasets despite still opening directly in Excel rather than a bare file-save dialog.