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)

8 files changed, 56 insertions, 38 deletions

diff --git a/lib/axlsx.rb b/lib/axlsx.rb
index cf3beb87..09f351fc 100644
--- a/lib/axlsx.rb
+++ b/lib/axlsx.rb
@@ -63,4 +63,25 @@ module Axlsx
     [v[:i]-1, ((name[/[1-9][0-9]*/]).to_i)-1]
 
   end
+
+  # converts the column index into alphabetical values.
+  # @note This follows the standard spreadsheet convention of naming columns A to Z, followed by AA to AZ etc.
+  # @return [String]
+  def self.col_ref(index)
+    chars = []
+    while index >= 26 do
+      chars << ((index % 26) + 65).chr
+      index /= 26
+    end
+    chars << ((chars.empty? ? index : index-1) + 65).chr
+    chars.reverse.join
+  end
+
+  # @return [String] The alpha(column)numeric(row) reference for this sell.
+  # @example Relative Cell Reference
+  #   ws.rows.first.cells.first.r #=> "A1"
+  def self.cell_r(c_index, r_index)
+      Axlsx::col_ref(c_index).to_s << (r_index+1).to_s
+    end
+
 end
diff --git a/lib/axlsx/stylesheet/color.rb b/lib/axlsx/stylesheet/color.rb
index 5d3e60ca..2a08fd3c 100644
--- a/lib/axlsx/stylesheet/color.rb
+++ b/lib/axlsx/stylesheet/color.rb
@@ -62,12 +62,11 @@ module Axlsx
     # def indexed=(v) Axlsx::validate_unsigned_integer v; @indexed = v end
 
     def to_xml_string
-      str = ["<color "]
+      str = "<color "
       self.instance_values.each do |key, value|
-        str << "%s='%s' " % [key, value]
+        str << key << '="' << value.to_s << '" '
       end
       str << "/>"
-      str.join
     end
 
     # Serializes the color
diff --git a/lib/axlsx/workbook/shared_strings_table.rb b/lib/axlsx/workbook/shared_strings_table.rb
index 5ebfd87f..0bdd7936 100644
--- a/lib/axlsx/workbook/shared_strings_table.rb
+++ b/lib/axlsx/workbook/shared_strings_table.rb
@@ -32,11 +32,12 @@ module Axlsx
       cells = cells.flatten.reject { |c| c.type != :string || c.value.start_with?('=') }
       @count = cells.size
       @unique_cells = []
+      @shared_xml_string = ""
       resolve(cells)
     end
 
     def to_xml_string
-      str = "<sst xmlns=\"%s\" count='%s' uniqueCount='%s'>%s</sst>" % [XML_NS, count, unique_count, @unique_cells.map {|cell| cell[:data]}.join]
+      '<sst xmlns="' << XML_NS << '" count="' << @count.to_s << '" uniqueCount="' << unique_count.to_s << '">' << @shared_xml_string << '</sst>'
     end
 
     # Generate the xml document for the Shared Strings Table
@@ -64,11 +65,12 @@ module Axlsx
       cells.each do |cell|
         cell_hash = cell.shareable_hash
         index = @unique_cells.index do |item|
-          item[:hash] == cell_hash
+          item == cell_hash
         end
         if index == nil
           cell.send :ssti=, @unique_cells.size
-          @unique_cells << {:hash => cell_hash, :data => '<si>'<< cell.run_xml_string << '</si>'}
+          @shared_xml_string << '<si>' << cell.run_xml_string << '</si>'
+          @unique_cells << cell_hash
         else
           cell.send :ssti=, index
         end
diff --git a/lib/axlsx/workbook/worksheet/cell.rb b/lib/axlsx/workbook/worksheet/cell.rb
index a3ec046a..24b380cb 100644
--- a/lib/axlsx/workbook/worksheet/cell.rb
+++ b/lib/axlsx/workbook/worksheet/cell.rb
@@ -238,8 +238,9 @@ module Axlsx
     # @return [String] The alpha(column)numeric(row) reference for this sell.
     # @example Relative Cell Reference
     #   ws.rows.first.cells.first.r #=> "A1"
+    # @note this will be discontinued in 1.1.0 -  prefer Axlsx.cell_r
     def r
-      "#{col_ref}#{@row.index+1}"
+      "#{Axlsx::col_ref(index)}#{@row.index+1}"
     end
 
     # @return [String] The absolute alpha(column)numeric(row) reference for this sell.
@@ -275,9 +276,9 @@ module Axlsx
       self.row.worksheet.merge_cells "#{self.r}:#{range_end}" unless range_end.nil?
     end
 
-    def run_xml_string
-      str = ""
+    def run_xml_string(str = '')
       if is_text_run?
+        puts 'text run'
         data = self.instance_values.reject{|key, value| value == nil }
         keys = data.keys & INLINE_STYLES
         keys.delete ['value', 'type']
@@ -333,30 +334,32 @@ module Axlsx
     # Serializes the cell
     # @param [Nokogiri::XML::Builder] xml The document builder instance this objects xml will be added to.
     # @return [String] xml text for the cell
-    def to_xml_string
+    def to_xml_string(r_index, c_index, str = '')
+      str << '<c r="' << Axlsx::cell_r(c_index, r_index) << '" s="' << @style.to_s << '" '
       case @type
       when :string
         #parse formula
         if @value.start_with?('=')
-          '<c r="' << r << '" t="str" s="' << @style.to_s << '"><f>' << value.to_s.gsub('=', '') << '</f></c>'
+          str  << 't="str"><f>' << value.to_s.gsub('=', '') << '</f>'
         else
           #parse shared
           if @ssti
-            '<c r="' << r << '" t="s" s="' << @style.to_s << '"><v>' << ssti << '</v></c>'
+            str << 't="s"><v>' << ssti << '</v>'
           else
-            '<c r="' << r << '" t="inlineStr" s="' << @style.to_s << '"><is>' << run_xml_string << '</is></c>'
+            str << 't="inlineStr"><is>' << run_xml_string << '</is>'
           end
         end
       when :date
         # TODO: See if this is subject to the same restriction as Time below
-        '<c r="' << r << '" s="' << @style.to_s << '"><v>' << DateTimeConverter::date_to_serial(@value).to_s << '</v></c>'
+        str << '><v>' << DateTimeConverter::date_to_serial(@value).to_s << '</v>'
       when :time
-        '<c r="' << r << '" s="' << @style.to_s << '"><v>' << DateTimeConverter::time_to_serial(@value).to_s << '</v></c>'
+        str << '><v>' << DateTimeConverter::time_to_serial(@value).to_s << '</v>'
       when :boolean
-        '<c r="' << r << '" t="b" s="' << @style.to_s << '"><v>' << @value.to_s << '</v></c>'
+        str << 't="b"><v>' << @value.to_s << '</v>'
       else
-        '<c r="' << r << '" s="' << @style.to_s << '"><v>' << @value.to_s << '</v></c>'
+        str << '><v>' << @value.to_s << '</v>'
       end
+      str << '</c>'
     end
 
     def to_xml(xml)
@@ -412,20 +415,6 @@ module Axlsx
     # assigns the owning row for this cell.
     def row=(v) DataTypeValidator.validate "Cell.row", Row, v; @row=v end
 
-    # converts the column index into alphabetical values.
-    # @note This follows the standard spreadsheet convention of naming columns A to Z, followed by AA to AZ etc.
-    # @return [String]
-    def col_ref
-      chars = []
-      index = self.index
-      while index >= 26 do
-        chars << ((index % 26) + 65).chr
-        index /= 26
-      end
-      chars << ((chars.empty? ? index : index-1) + 65).chr
-      chars.reverse.join
-    end
-
     # Determines the cell type based on the cell value.
     # @note This is only used when a cell is created but no :type option is specified, the following rules apply:
     #   1. If the value is an instance of Date, the type is set to :date
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.
diff --git a/lib/axlsx/workbook/worksheet/worksheet.rb b/lib/axlsx/workbook/worksheet/worksheet.rb
index 6a029f19..20c13d8b 100644
--- a/lib/axlsx/workbook/worksheet/worksheet.rb
+++ b/lib/axlsx/workbook/worksheet/worksheet.rb
@@ -386,7 +386,9 @@ module Axlsx
         str.concat '</cols>'
       end
 
-      str.concat "<sheetData>%s</sheetData>" % @rows.map { |obj| obj.to_xml_string }.join
+      str.concat '<sheetData>'
+      @rows.each_with_index { |row, index| row.to_xml_string(index, str) }
+      str.concat '</sheetData>'
       str.concat page_margins.to_xml_string if @page_margins
       str.concat "<autoFilter ref='%s'></autoFilter>" % @auto_filter if @auto_filter
       str.concat "<mergeCells count='%s'>%s</mergeCells>" % [@merged_cells.size, @merged_cells.reduce('') { |memo, obj| "<mergeCell ref='%s'></mergeCell>" % obj } ] unless @merged_cells.empty?
diff --git a/test/workbook/worksheet/tc_cell.rb b/test/workbook/worksheet/tc_cell.rb
index 991181e7..499f9209 100644
--- a/test/workbook/worksheet/tc_cell.rb
+++ b/test/workbook/worksheet/tc_cell.rb
@@ -69,7 +69,8 @@ class TestCell < Test::Unit::TestCase
   end
 
   def test_col_ref
-    assert_equal(@c.send(:col_ref), "A")
+    #TODO move to axlsx spec
+    assert_equal(Axlsx.col_ref(0), "A")
   end
 
   def test_cell_type_from_value
diff --git a/test/workbook/worksheet/tc_row.rb b/test/workbook/worksheet/tc_row.rb
index b77715c5..47b0f054 100644
--- a/test/workbook/worksheet/tc_row.rb
+++ b/test/workbook/worksheet/tc_row.rb
@@ -61,14 +61,14 @@ class TestRow < Test::Unit::TestCase
   end
 
   def test_to_xml_string
-    r_s_xml = Nokogiri::XML(@row.to_xml_string)
+    r_s_xml = Nokogiri::XML(@row.to_xml_string(0, ''))
     assert_equal(r_s_xml.xpath(".//row[@r=1]").size, 1)
   end
 
   def test_to_xml_string_with_custom_height
     @row.add_cell 1
     @row.height = 20
-    r_s_xml = Nokogiri::XML(@row.to_xml_string)
+    r_s_xml = Nokogiri::XML(@row.to_xml_string(0, ''))
     assert_equal(r_s_xml.xpath(".//row[@r=1][@ht=20][@customHeight=1]").size, 1)
   end