If there's a conversion step that is unnecessary in your case, you can perhaps add a flag somewhere to explicitly turn it off (but I suggest that you make some actual measurements of what you'll gain with it first, since it isn't unlikely that there are much bigger cpu hoggers elsewhere).
Anyway, either it UTF8 encodes completely or it don't do it at all. Anything else is invariably bogus (which reminds me of the wml-url encoding in Roxen which is bogus in exactly this way - I still like to get some sort of answer from Nilsson on that).
Well, come to think of it, a third option is actually to do it conditionally and also store a flag which tells whether it was UTF8 encoded or not. In any case, when it's time to decode the string you have to _know_ if it's to be UTF8 decoded or not.