Still not fast enough?

```
                        user     system      total        real                                                               
axlsx_noautowidth   0.760000   0.020000   0.780000 (  0.885482)                                                              
axlsx               3.560000   0.130000   3.690000 (  4.158594)                                                              
axlsx_shared       11.610000   0.180000  11.790000 ( 13.208945)                                                             
axlsx_stream        3.450000   0.120000   3.570000 (  3.920745)                                                              
csv                 0.240000   0.010000   0.250000 (  0.269822)

1 files changed, 7 insertions, 3 deletions

diff --git a/lib/axlsx/workbook/worksheet/row.rb b/lib/axlsx/workbook/worksheet/row.rb
index 30101a28..ac251ed8 100644
--- a/lib/axlsx/workbook/worksheet/row.rb
+++ b/lib/axlsx/workbook/worksheet/row.rb
@@ -59,12 +59,16 @@ module Axlsx
       worksheet.rows.index(self)
     end
 
-    def to_xml_string
+    def to_xml_string(r_index, str = '')
+      str << '<row r="' << (r_index + 1 ).to_s << '" '
       if custom_height?
-        '<row r="' << (index+1).to_s << '" customHeight="1" ht="' << height.to_s << '">' << @cells.map { |cell| cell.to_xml_string }.join << '</row>'
+        str << 'customHeight="1" ht="' << height.to_s << '">'
       else
-        '<row r="' << (index+1).to_s << '">' << (@cells.map { |cell| cell.to_xml_string }.join) << '</row>'
+        str << '>'
       end
+      @cells.each_with_index { |cell, c_index| cell.to_xml_string(r_index, c_index, str) }
+      str << '</row>'
+      str
     end
     # Serializes the row
     # @param [Nokogiri::XML::Builder] xml The document builder instance this objects xml will be added to.