Faster rendering by less split

lib/hexo/post.js

@@ -418,15 +418,37 @@ class Post {
         toString: true
       }, options);
     }).then(content => {
+      // This function restores swig tags in `content` and render them.
+      if (disableNunjucks) {
+        // If rendering is disabled, do nothing.
+        return cacheObj.restoreAllSwigTags(content);
+      }
+      // We'd like to render tags concurrently, so we split `content`
+      // by top-level HTML nodes that have swig tags into `split_content` array
+      // (nodes that don't have swig tags don't need to be split).
+      // Then we render items in `split_content` asynchronously.
       const doc = parse5.parseFragment(content);
-      // Indepedently render the tags in each top-level node
-      // asynchronously. This can significantly speed up rendering of slow tags
-      const results = doc.childNodes.map(async node => {
-        // Replace cache data with real contents
-        const content = cacheObj.restoreAllSwigTags(parse5.serializeOuter(node));
-        // If disabled, return content after replacing the placeholders
-        if (disableNunjucks) return content;
-        // Render with Nunjucks
+      const split_content = [];
+      let current = []; // Current list of nodes to be added to split_content.
+      doc.childNodes.forEach(node => {
+        const html = parse5.serializeOuter(node);
+        const restored = cacheObj.restoreAllSwigTags(html);
+        current.push(restored);
+        if (html !== restored) {
+          // Once we encouner a node that has swig tags, merge
+          // all content we've seen so far and add to `split_content`.
+          // We don't simply add every node to `split_content`, because
+          // most nodes don't have swig tags and calling `tag.render` for
+          // all of them has significant overhead.
+          split_content.push(current.join(''));
+          current = [];
+        }
+      });
+      if (current.length) {
+        split_content.push(current.join(''));
+      }
+      // Render the tags in each top-level node asynchronously.
+      const results = split_content.map(async content => {
         return await tag.render(content, data);
       });
       return Promise.all(results).then(x => x.join(''));