Let's be realistic here: Code wrapping graphic toolkits will not be a hotspot. Any heavy graphical computation is likely to be done in the graphic toolkit (or rendering engine, etc.) and called from Racket. That's the whole point.
I think your advice holds in general, but one interesting exception can be implementing a model, in the MVC sense. This can require a lot of communication across the language wrapping, and definitely become a hotspot. This is less of a problem in toolkits that retain state, but then you pay a cost in maintaining duplicate representations.