How to speed up autosizing columns in apache POI?

The autoSizeColumn function itself works not perfect and some columns width not exactly fit the data inside. So, I found some solution that works for me.

1. To avoid crazy calculations let give that to autoSizeColumn() function:

   sheet.autoSizeColumn(<columnIndex>);

2. Now, our column autosized by library but we wont to add a little bit more to the current column width to make table looks fine:

   // get autosized column width
   int currentColumnWidth = sheet.getColumnWidth(<columnIndex>);

   // add custom value to the current width and apply it to column
   sheet.setColumnWidth(<columnIndex>, (currentColumnWidth + 2500));

3. The full function could looks like:

   public void autoSizeColumns(Workbook workbook) {
        int numberOfSheets = workbook.getNumberOfSheets();
        for (int i = 0; i < numberOfSheets; i++) {
            Sheet sheet = workbook.getSheetAt(i);
            if (sheet.getPhysicalNumberOfRows() > 0) {
                Row row = sheet.getRow(sheet.getFirstRowNum());
                Iterator<Cell> cellIterator = row.cellIterator();
                while (cellIterator.hasNext()) {
                    Cell cell = cellIterator.next();
                    int columnIndex = cell.getColumnIndex();
                    sheet.autoSizeColumn(columnIndex);
                    int currentColumnWidth = sheet.getColumnWidth(columnIndex);
                    sheet.setColumnWidth(columnIndex, (currentColumnWidth + 2500));
                }
            }
        }
    }

P.S. Thanks Ondrej Kvasnovsky for the function https://stackoverflow.com/a/35324693/13087091

Solution which worked for me:

It was possible to avoid merged regions, so I could iterate through the other cells and finally autosize to the largest cell like this:

int width = ((int)(maxNumCharacters * 1.14388)) * 256;
sheet.setColumnWidth(i, width);

where 1.14388 is a max character width of the "Serif" font and 256 font units.

Performance of autosizing improved from 10 minutes to 6 seconds.