diff --git a/changelog.html b/changelog.html
index ae30b3a974e112e6425b9085ca0dbab72565673f..d59202fe006aa8f1b2c487589a0009f25f16292e 100644
--- a/changelog.html
+++ b/changelog.html
@@ -58,6 +58,9 @@ Upcoming changes</a>
   <li class=bug>
     Expensive symlink-related calls on Windows can be simplified.
     (<a href="https://issues.jenkins-ci.org/browse/JENKINS-20534">issue 20534</a>)
+  <li class=bug>
+    Slow rendering of directories with many entries in remote workspaces.
+    (<a href="https://issues.jenkins-ci.org/browse/JENKINS-21780">issue 21780</a>)
 </ul>
 </div><!--=TRUNK-END=-->
 
diff --git a/core/src/main/java/hudson/model/DirectoryBrowserSupport.java b/core/src/main/java/hudson/model/DirectoryBrowserSupport.java
index bfc6d917da53bb845f969f96d0479ae60273b9e7..1dfd2b3a3b3993b68fec9922df76c6fe5036f176 100644
--- a/core/src/main/java/hudson/model/DirectoryBrowserSupport.java
+++ b/core/src/main/java/hudson/model/DirectoryBrowserSupport.java
@@ -25,14 +25,6 @@ package hudson.model;
 
 import hudson.FilePath;
 import hudson.Util;
-import jenkins.model.Jenkins;
-import org.apache.commons.io.IOUtils;
-import org.kohsuke.stapler.StaplerRequest;
-import org.kohsuke.stapler.StaplerResponse;
-import org.kohsuke.stapler.HttpResponse;
-
-import javax.servlet.ServletException;
-import javax.servlet.http.HttpServletResponse;
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
@@ -42,14 +34,23 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.Comparator;
+import java.util.HashMap;
 import java.util.List;
 import java.util.Locale;
+import java.util.Map;
 import java.util.StringTokenizer;
-import java.util.logging.Logger;
 import java.util.logging.Level;
+import java.util.logging.Logger;
 import java.util.zip.ZipEntry;
 import java.util.zip.ZipOutputStream;
+import javax.servlet.ServletException;
+import javax.servlet.http.HttpServletResponse;
+import jenkins.model.Jenkins;
 import jenkins.util.VirtualFile;
+import org.apache.commons.io.IOUtils;
+import org.kohsuke.stapler.HttpResponse;
+import org.kohsuke.stapler.StaplerRequest;
+import org.kohsuke.stapler.StaplerResponse;
 
 /**
  * Has convenience methods to serve file system.
@@ -430,8 +431,9 @@ public final class DirectoryBrowserSupport implements HttpResponse {
 
     private static final class FileComparator implements Comparator<VirtualFile> {
         private Collator collator;
+        private final Map<VirtualFile,Boolean> isDirCache = new HashMap<VirtualFile,Boolean>();
 
-        public FileComparator(Locale locale) {
+        FileComparator(Locale locale) {
             this.collator = Collator.getInstance(locale);
         }
 
@@ -443,9 +445,18 @@ public final class DirectoryBrowserSupport implements HttpResponse {
             return this.collator.compare(lhs.getName(), rhs.getName());
         }
 
+        private boolean isDirectory(VirtualFile f) throws IOException {
+            Boolean known = isDirCache.get(f);
+            if (known == null) {
+                known = f.isDirectory();
+                isDirCache.put(f, known);
+            }
+            return known;
+        }
+
         private int dirRank(VirtualFile f) {
             try {
-            if(f.isDirectory())     return 0;
+            if(isDirectory(f))     return 0;
             else                    return 1;
             } catch (IOException ex) {
                 return 0;