The VWO SmartCode snippet that you add to your pages contacts VWO server whenever a visitor lands on a test page. The server sends its response in two parts - static JavaScript code for running the test and dynamic test-specific data. Both the static and dynamic parts of the response are cached heavily at the server back-end, thereby minimizing the response time. With code minification and gzipping, the size of response decreases even further.
To understand more about VWO SmartCode, read our knowledge base article on VWO Accounts.