async is better

Without any async and defer

• Script tag without attributes will parse HTML till it hits the script tag
• then download the script, parse and execute it
• Resume parsing HTML

Async

• Downloads the script in parallel to HTML parsing,
• When script is available, pause the parsing, execute the script
• Order of execution of scripts is not guaranteed, as script will execute as its available

Defer

• Downloads the script in parallel to HTML parsing
• Execute scripts after the HTML parsing,
• Order of execution of scripts is guaranteed.

Both async and defer

• Browsers which supports async, execute as async
• Old browsers executes as defer.