Ribbon works great in SharePoint 2010. It uses all JavaScript and Client OM to render the ribbon components. If you are not adding a lot of server side controls in the ribbon it will definitely make your site faster. Now let’s talk about Some of the methods that Ribbon uses to improve the site Performance .

Cashing - The Out-of-Box ribbon in heavily Cashed. So the controls are downloaded once and are used for the rest of the site.

Client OM - In terms of Architecture, ribbon Xml defines the UI but the behavior is all controlled by the JavaScript. You can definitely have Client or server controls in the ribbon according to your need.

CSS UI - Ribbon has all Css layout for UI and does not uses any tables.

Clustered Images - All the Images displayed in the ribbon like icons and stuff are Clustered images, thus they reduce the round-trip to the server.

Dynamic scaling and Pop-ups - All the pop-ups are displayed dynamically.

For more Please see : Ribbon in SharePoint 2010