It's difficult to answer.
You can always measure wrong, interpret values wrong, or do not repeat tests correctly.
You must trust the DevTools of your chosen browser. Just adding JS code to measure DOM performance or CSS performance is stressing the wrong part of your browser.
I check manually if things feel smooth. If it feels laggy I spin up Chrome/Firefox DevTools and look for performance graphs. 'Paint' and 'Layout' are the bad guys to look for. To start with good performance, I often look at csstriggers.com to find which property to avoid.