It depends what I'm working on. If it's a bunch of interdependent systems that involve a large amount of data, a giant monitor is better. If the giant monitor is being used to make visible more application surfaces (Slack, email, VS Code, etc.), it makes focus worse.

The biggest improvement I've found for my focus is to force myself to close any open tabs/windows that are not absolutely necessary roughly every two hours. I used to be one of those people with 800 tabs open in the browser and 20 application windows spread across 8 desktop spaces. Was a concentration mess. Requiring myself to "clean up" periodically has really helped.