diff --git a/WHATSNEW b/WHATSNEW
index a8b6c9119..52c94397e 100644
--- a/WHATSNEW
+++ b/WHATSNEW
@@ -179,6 +179,10 @@ Other changes:
    an SSH session providing (local and remote) tunnels.
    Bugzilla Issue 43083.
 
+ * ZipOutputStream has been sped up for certain usage scenarios that
+   are not used by Ant's family of zip tasks.
+   Bugzilla report 45396.
+
 Changes from Ant 1.7.0 TO Ant 1.7.1
 =============================================
 
diff --git a/src/main/org/apache/tools/zip/ZipOutputStream.java b/src/main/org/apache/tools/zip/ZipOutputStream.java
index 765728540..4b03d005e 100644
--- a/src/main/org/apache/tools/zip/ZipOutputStream.java
+++ b/src/main/org/apache/tools/zip/ZipOutputStream.java
@@ -58,6 +58,14 @@ public class ZipOutputStream extends FilterOutputStream {
     private static final int SHORT = 2;
     private static final int WORD = 4;
     private static final int BUFFER_SIZE = 512;
+    /* 
+     * Apparently Deflater.setInput gets slowed down a lot on Sun JVMs
+     * when it gets handed a really big buffer.  See
+     * https://issues.apache.org/bugzilla/show_bug.cgi?id=45396
+     *
+     * Using a buffer size of 8 kB proved to be a good compromise
+     */
+    private static final int DEFLATER_BLOCK_SIZE = 8192;
 
     /**
      * Compression method for deflated entries.
@@ -482,9 +490,21 @@ public class ZipOutputStream extends FilterOutputStream {
         if (entry.getMethod() == DEFLATED) {
             if (length > 0) {
                 if (!def.finished()) {
-                    def.setInput(b, offset, length);
-                    while (!def.needsInput()) {
-                        deflate();
+                    if (length <= DEFLATER_BLOCK_SIZE) {
+                        def.setInput(b, offset, length);
+                        deflateUntilInputIsNeeded();
+                    } else {
+                        final int fullblocks = length / DEFLATER_BLOCK_SIZE;
+                        for (int i = 0; i < fullblocks; i++) {
+                            def.setInput(b, offset + i * DEFLATER_BLOCK_SIZE,
+                                         DEFLATER_BLOCK_SIZE);
+                            deflateUntilInputIsNeeded();
+                        }
+                        final int done = fullblocks * DEFLATER_BLOCK_SIZE;
+                        if (done < length) {
+                            def.setInput(b, offset + done, length - done);
+                            deflateUntilInputIsNeeded();
+                        }
                     }
                 }
             }
@@ -915,4 +935,10 @@ public class ZipOutputStream extends FilterOutputStream {
         }
     }
 
+    private void deflateUntilInputIsNeeded() throws IOException {
+        while (!def.needsInput()) {
+            deflate();
+        }
+    }
+
 }