diff -rupN -x '.*' poi-3.9/src/java/org/apache/poi/hssf/usermodel/HSSFCellStyle.java poi-3.9-patch/src/java/org/apache/poi/hssf/usermodel/HSSFCellStyle.java
--- poi-3.9/src/java/org/apache/poi/hssf/usermodel/HSSFCellStyle.java	2012-11-25 13:27:35.000000000 +0100
+++ poi-3.9-patch/src/java/org/apache/poi/hssf/usermodel/HSSFCellStyle.java	2013-10-19 19:14:48.000000000 +0200
@@ -18,9 +18,13 @@
 
 package org.apache.poi.hssf.usermodel;
 
+import java.util.Arrays;
+import java.util.List;
+
 import org.apache.poi.hssf.model.InternalWorkbook;
 import org.apache.poi.hssf.record.ExtendedFormatRecord;
 import org.apache.poi.hssf.record.FontRecord;
+import org.apache.poi.hssf.record.FormatRecord;
 import org.apache.poi.hssf.record.StyleRecord;
 import org.apache.poi.hssf.util.HSSFColor;
 import org.apache.poi.ss.usermodel.CellStyle;
@@ -106,8 +110,28 @@ public final class HSSFCellStyle impleme
      * @see org.apache.poi.hssf.usermodel.HSSFDataFormat
      * @return the format string or "General" if not found
      */
+     
+    private static short lastDateFormat = Short.MIN_VALUE;
+    private static List<FormatRecord> lastFormats = null;
+    private static String getDataFormatStringCache = null;
+    
+    private static final Object cacheLock = new Object();
+    
     public String getDataFormatString() {
-        return getDataFormatString(_workbook);
+       synchronized (cacheLock) {
+          if (getDataFormatStringCache != null) {
+             if (lastDateFormat == getDataFormat() && _workbook.getFormats().equals(lastFormats)) {
+        	       return getDataFormatStringCache;
+             }
+          }
+      
+          lastFormats = _workbook.getFormats();
+          lastDateFormat = getDataFormat();
+      
+          getDataFormatStringCache = getDataFormatString(_workbook);
+  	    
+  	       return getDataFormatStringCache;
+  	    }
     }
     /**
      * Get the contents of the format string, by looking up
@@ -828,6 +852,13 @@ public final class HSSFCellStyle impleme
 
     	// Handle matching things if we cross workbooks
     	if(_workbook != source._workbook) {
+    	   
+    	   synchronized (cacheLock) {
+            lastDateFormat = Short.MIN_VALUE;
+            lastFormats = null;
+            getDataFormatStringCache = null;
+    	   }
+    	   
 			// Then we need to clone the format string,
 			//  and update the format record for this
     		short fmt = (short)_workbook.createFormat(source.getDataFormatString() );
@@ -874,4 +905,5 @@ public final class HSSFCellStyle impleme
 		}
 		return false;
 	}
+	
 }