I don't know what thumbnail you are referring to, but if it is the one that is saved with assets, then there is an issue. One of the complaints has been that saving in iClone 6 took significantly longer than in IC5, as often more than half the time was spent rendering the thumbnail, even with a high end card. So IC 6 has now a less fancy option, which has improved the time needed for saving. A high-end thumbnail could be an option for those who want it, but it should be just that, an option.
If this is not what you mean then I'm sorry. Please, enlighten me.