To understand this better, let’s compare some key areas that make the WordPress Site Editor (Full Site Editing or FSE) and Classic Themes architecturally different.

Rendering Model

• Classic Themes: Primarily, they rely on server-side rendering of PHP files. Specifically, a Classic Theme is a combination of multiple structured .php (header.php, footer.php, single.php, etc.) files, collectively called the Template Hierarchy. The system uses WordPress Core to execute the logic written inside the theme to render output.
  
• Site Editor (Block Theme): Essentially, Block Themes use HTML templates composed of blocks. Once a page is requested, WordPress parses them internally at runtime internally and subsequently renders them via block definitions through PHP and JavaScript.

Template Structure and Hierarchy

• Classic Themes: A Classic Theme contains multiple PHP files, that are known as template files or simpley templates with conditional logic distributed among them. Some essential template files in a typical Template Hierarchy are index.php, header.php, footer.php, single.php, and sidebar.php. Classic Themes tightly bind the logic and the layout at the same place.
  
• Site Editor: Unlike the Classic Themes, here the templates are .html files with Block markup, while Core Blocks, Block Callbacks, and theme.json hold the core theme logic. Thus a Block Theme separates layout definition from the actual execution.
  

Data Flow & Control

• Classic Themes control both markup and logic from the heart of its template hierarchy. On the other hand, WordPress takes are of the logic for a Block theme. The theme part controls the structure, layout, and the style.

Customization Layer

• Classic Themes: In Classic Themes, PHP options, theme settings page, and Customizer control the customization. The customization in a Classic Themes is per-theme basis.
  
  
• Site Editor: A Block Theme, however, controls the customization through theme.json, Global styles and Block-level controls. It does a good amount of heavy lifting of configurational areas.

Performance

• Classic Themes: PHP template files render the pages directly. This ensures no or very little intervention between the request and the HTML output. In addition to that, they are cache-friendly for performance optimization.
  
  However, there is always a risk for a template loading more data than actually needed. Not only that, poorly structured queries can make the website vulnerable to SQL injection and other means of attacks. Security Best Practices must be followed and implemented to seal the sneaking points.
  
  
• Site Editor: WordPress uses parsing block templates and block markups to assemble the pages. The final rendering involves interpreting blocks, attributes, and styles. One of the major advantages of Block Templates is reusable layouts. This means the same block structure can be used across pages. It also ensures uniform HTML output throughout the website. However, there are a couple of small caveats too likeDOM complexity, Block parsing overhead, additional CSS and JavaScript enqueuing.