diff --git a/src/Symfony/Component/DomCrawler/Crawler.php b/src/Symfony/Component/DomCrawler/Crawler.php
index 3627aad5a1..b6ccda1372 100644
--- a/src/Symfony/Component/DomCrawler/Crawler.php
+++ b/src/Symfony/Component/DomCrawler/Crawler.php
@@ -33,6 +33,11 @@ class Crawler implements \Countable, \IteratorAggregate
      */
     private $namespaces = [];
 
+    /**
+     * @var \ArrayIterator A map of cached namespaces
+     */
+    private $cachedNamespaces;
+
     /**
      * @var string The base href value
      */
@@ -68,6 +73,7 @@ class Crawler implements \Countable, \IteratorAggregate
         $this->uri = $uri;
         $this->baseHref = $baseHref ?: $uri;
         $this->html5Parser = class_exists(HTML5::class) ? new HTML5(['disable_html_ns' => true]) : null;
+        $this->cachedNamespaces = new \ArrayIterator();
 
         $this->add($node);
     }
@@ -99,6 +105,7 @@ class Crawler implements \Countable, \IteratorAggregate
     {
         $this->nodes = [];
         $this->document = null;
+        $this->cachedNamespaces = new \ArrayIterator();
     }
 
     /**
@@ -967,12 +974,14 @@ class Crawler implements \Countable, \IteratorAggregate
      */
     private function filterRelativeXPath(string $xpath): object
     {
-        $prefixes = $this->findNamespacePrefixes($xpath);
-
         $crawler = $this->createSubCrawler(null);
+        if (null === $this->document) {
+            return $crawler;
+        }
+
+        $domxpath = $this->createDOMXPath($this->document, $this->findNamespacePrefixes($xpath));
 
         foreach ($this->nodes as $node) {
-            $domxpath = $this->createDOMXPath($node->ownerDocument, $prefixes);
             $crawler->add($domxpath->query($xpath, $node));
         }
 
@@ -1189,10 +1198,17 @@ class Crawler implements \Countable, \IteratorAggregate
             return $this->namespaces[$prefix];
         }
 
+        if ($this->cachedNamespaces->offsetExists($prefix)) {
+            return $this->cachedNamespaces->offsetGet($prefix);
+        }
+
         // ask for one namespace, otherwise we'd get a collection with an item for each node
         $namespaces = $domxpath->query(sprintf('(//namespace::*[name()="%s"])[last()]', $this->defaultNamespacePrefix === $prefix ? '' : $prefix));
 
-        return ($node = $namespaces->item(0)) ? $node->nodeValue : null;
+        $namespace = ($node = $namespaces->item(0)) ? $node->nodeValue : null;
+        $this->cachedNamespaces->offsetSet($prefix, $namespace);
+
+        return $namespace;
     }
 
     private function findNamespacePrefixes(string $xpath): array
@@ -1217,6 +1233,7 @@ class Crawler implements \Countable, \IteratorAggregate
         $crawler->isHtml = $this->isHtml;
         $crawler->document = $this->document;
         $crawler->namespaces = $this->namespaces;
+        $crawler->cachedNamespaces = $this->cachedNamespaces;
         $crawler->html5Parser = $this->html5Parser;
 
         return $crawler;