Why does Safari start so quickly on OS X compared to other browsers?

Apple works extremely hard to ensure that their apps adhere to the same rules that they expect everyone else to adhere to. It turns out that employing such APIs speeds up and improves efficiency. Because the Safari team only supports one OS, this is considerably easier for them to do.

• The overwhelming popularity of Google Chrome on macOS is impressive for a non-default browser, but it makes sense. Chrome has a reputation for being light and quick in its early days. People said it was better than Safari and Firefox. It may have been true at the time, but it is no longer true.
• In reality, Safari on the Mac outperforms Chrome in terms of energy efficiency, privacy protection, and compatibility with the Apple ecosystem. 

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