I've almost started investigating performance issues a few times because i've been tripped up by being in theme designer mode and using the more theme and wondered why things are going so slow. (The reason which I am well aware of is because of the slow dynamic compiling of less).

It would be good to add something obvious to the performance info footer to make it obvious that this is impacting the responsiveness of the page. Today when I experienced it I only twigged why it was happening because I saw the themedesignermode adhoc cache (and i'm really familiar this problem).

To someone who doesn't know about this gotcha it would be less obvious.

I am not sure if its possible to work out from the php page that less is being compiled on the fly, so perhaps theme designer mode is the best we can do (I still think thats useful because it'll warn you that normal caching isn't taking place). But it'd be great if we could indicate that a less compilation is happening too (it helps explain when slower than normal after a theme setting change)