Not sending the rounded corners will increase page rendering time in browsers that do support rounded borders.

It will not actually improve users time that it takes to get the page from your server.

The rounded corners are rendered client side as compwhizii points out. True, the overall performance of board will improve, if the client render time is reduced. On a modern machine, however, this change will have almost no effect. This is a technicality.

What I'd like to see are perf comparisons on neutral hardware using the same payload.