The best way to feel the actual difference with and without Speed Kit is as follows:
In Chrome: Press F12 to open up the developer console. Navigate to the "Application"-tab and search "Service Workers" on the left side. On the right side, you have a checkbox „Bypass for network“. By checking you can disable Speed Kit for you as long as the developer console is open.
Navigate the page with this option checked and unchecked to feel the difference.
Please note that performance feels different for different devices, users and network connections. To see the full picture as well as the benefits from the performance improvement, you need to look at real user data in an A/B Test.