diff --git a/core/lib/Drupal/Core/Lock/LockBackendAbstract.php b/core/lib/Drupal/Core/Lock/LockBackendAbstract.php
index 41ee27cab4..32e5ab4c6a 100644
--- a/core/lib/Drupal/Core/Lock/LockBackendAbstract.php
+++ b/core/lib/Drupal/Core/Lock/LockBackendAbstract.php
@@ -37,18 +37,30 @@ public function wait($name, $delay = 30) {
     // time until it reaches 500ms. After this point polling will continue every
     // 500ms until $delay is reached.
 
+    // If executing inside a fiber, then suspending the fiber implicitly waits
+    // for whatever the parent process does before it is resumed again. Try
+    // that so that other tasks can continue, before sleeping only if necessary.
+    if (\Fiber::getCurrent() !== NULL) {
+      $sleep = 0;
+      \Fiber::suspend();
+    }
+    else {
+      // Begin sleeping at 25ms.
+      $sleep = 25000;
+    }
+
     // $delay is passed in seconds, but we will be using usleep(), which takes
     // microseconds as a parameter. Multiply it by 1 million so that all
     // further numbers are equivalent.
     $delay = (int) $delay * 1000000;
 
-    // Begin sleeping at 25ms.
-    $sleep = 25000;
     while ($delay > 0) {
       // This function should only be called by a request that failed to get a
-      // lock, so we sleep first to give the parallel request a chance to finish
-      // and release the lock.
-      usleep($sleep);
+      // lock, so if  we haven't already suspended a fiber, sleep first to give
+      // the parallel request a chance to finish and release the lock.
+      if ($sleep > 0) {
+        usleep($sleep);
+      }
       // After each sleep, increase the value of $sleep until it reaches
       // 500ms, to reduce the potential for a lock stampede.
       $delay = $delay - $sleep;
diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index f0e6118a39..adf1cde7d8 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -237,13 +237,19 @@ protected function init($theme_name = NULL) {
    */
   public function get() {
     $this->init($this->themeName);
-    if (isset($this->registry[$this->theme->getName()])) {
-      return $this->registry[$this->theme->getName()];
-    }
-    if ($cache = $this->cache->get('theme_registry:' . $this->theme->getName())) {
-      $this->registry[$this->theme->getName()] = $cache->data;
-    }
-    else {
+    if (!$this->cacheGet()) {
+      // If called from inside a fiber, suspend it, this may allow another code
+      // path to begin an asynchronous operation before we do the CPU-intensive
+      // task of building the theme registry.
+      if (\Fiber::getCurrent() !== NULL) {
+        \Fiber::suspend();
+        // When we return from the fiber being suspended, check the cache again
+        // since it may have been built in the meantime, either in this process
+        // or via a different request altogether.
+        if ($this->cacheGet()) {
+          return $this->registry[$this->theme->getName()];
+        }
+      }
       $this->build();
       // Only persist it if all modules are loaded to ensure it is complete.
       if ($this->moduleHandler->isLoaded()) {
@@ -253,6 +259,16 @@ public function get() {
     return $this->registry[$this->theme->getName()];
   }
 
+  /**
+   * Gets the theme registry cache.
+   */
+  protected function cacheGet(): void {
+    $theme_name = $this->theme->getName();
+    if (!isset($this->registry[$theme_name]) && $cache = $this->cache->get('theme_registry:' . $theme_name)) {
+      $this->registry[$theme_name] = $cache->data;
+    }
+  }
+
   /**
    * Returns the incomplete, runtime theme registry.
    *