[franklyt]

Firefox is not as performant as Chrome is. This is something a web developer will have come across, if not an average user.

[butz]

While Chrome performance is obviously better for browsing, somehow opening DevTools makes it work much slower. As for casual user, installing ad-blocker makes web browsing way faster.

[SquareWheel]

If you leave Disable Cache checked under the network tab, the web will be substantially slower with dev tools open. This is (likely) the default setting, because it's important for testing.