readability - Mixed case vs all lower case...Which is more readable?

A graphic designer I hired to do my company's collateral (a very accomplished and successful designer) told me once that all lower case is often more readable than mixed case. As an example she made my last name all lower case on my business card...my last name in standard form is DesLonde...on the business card she made it deslonde.

I have to admit, it DOES seem more readable.

I'm developing a productivity app where I want the efficiency to be as high as possible, every little distraction taken away, every inefficiency eliminated. I want it to be as easy to use as possible. Even a fraction of a second eliminated would be desired.

Would the app benefit from making button text all lower case, group headings all lower case, etc.?

Would it benefit from some parts being all lower case? Does it even help? Some help here from ux people who know more than me. :-) Thanks!