What can happen on the server during HTML (SSR) generation — what does the server spend time on, what can be problems with performance and security during data serialization?

When generating HTML on the server (SSR), the server performs several key tasks that take time:

• Component rendering: the server runs the application's JavaScript code to form the HTML structure. This may include calculations, API calls, data processing.
• Database and external service requests: to fill the page with data, the server may make requests to databases or other services, increasing response time.
• Data serialization: to transmit the application's state to the client, data is serialized (e.g., into JSON) so the client can use it for hydration.

Performance issues:

• Long database or API requests slow down page generation.
• Complex or deep component rendering increases CPU load.
• Large volumes of serializable data increase transmission and parsing time on the client.

Security issues with data serialization:

• XSS vulnerabilities: if serialized data is inserted into HTML without proper escaping, an attacker can inject scripts.
• Injections: improper data handling can lead to execution of unwanted code.

For security, it is important to use proven serialization and escaping methods, such as JSON.stringify with subsequent safe insertion into HTML, and to avoid inserting raw data into scripts without processing.