diff --git a/src/infrastructure/env/PhabricatorEnv.php b/src/infrastructure/env/PhabricatorEnv.php
--- a/src/infrastructure/env/PhabricatorEnv.php
+++ b/src/infrastructure/env/PhabricatorEnv.php
@@ -55,6 +55,7 @@
   private static $overrideSource;
   private static $requestBaseURI;
   private static $cache;
+  private static $cacheSelfDomains;
   private static $localeCode;
   private static $readOnly;
   private static $readOnlyReason;
@@ -455,15 +456,26 @@
 
     $host = phutil_utf8_strtolower($host);
 
-    $self_map = self::getSelfURIMap();
+    $self_map = self::getSelfURIMapCached();
     return isset($self_map[$host]);
   }
 
-  private static function getSelfURIMap() {
-    // Note: here we always re-build some PhutilURI(s), but this is not
-    // blazing fast, so, we could add a small cache for our result.
-    // https://we.phorge.it/T16061
+  /**
+   * Get the self URI map, cached.
+   * @return array Map of trusted domains, in both the key and the value.
+   */
+  private static function getSelfURIMapCached() {
+    if (!self::$cacheSelfDomains) {
+      self::$cacheSelfDomains = self::getSelfURIMap();
+    }
+    return self::$cacheSelfDomains;
+  }
 
+  /**
+   * Get the self URI map, uncached.
+   * @return array Map of trusted domains, in both the key and the value.
+   */
+  private static function getSelfURIMap() {
     $self_uris = array();
     $self_uris[] = self::getProductionURI('/');
     $self_uris[] = self::getURI('/');
@@ -976,6 +988,7 @@
 
   private static function dropConfigCache() {
     self::$cache = array();
+    self::$cacheSelfDomains = array();
   }
 
   private static function resetUmask() {