I thought it might be worth mentioning that Sitecore recommends that all non-technical users start in the simple Page Editor inline editing interface rather than in the more capable Content Editor or Desktop user interfaces, which are intended for advanced content managers, developers, and administrators. Also, you can use Sitecore Client Security Roles to limit the features available in all Sitecore user interfaces, which seriously reduces perceived complexity, but also controls which users can perform specific functions. You can expose more features as users become more familiar with the system and find needs for those features.

Sitecore has put a great deal of effort into maximizing client performance without sacrificing features or usability. The less data you send over the network, and the closer you are to the server, the better a Web application performs, which is another reason to use the Page Editor and limit features available in all of the user interfaces. Developers tend to turn on all sorts of the features that they only need occasionally, and forget to turn them off, but they can hide tabs, disable prefetching of collapsed sections (and collapse sections they use rarely), hide the standard fields, and take other steps to improve client performance.

Personally, I would always vote for features and usability over performance, but then again, I have never used Umbraco.