Why would that have to be the case? Also, wouldn't performance and thermals go together? You won't get good performance if you're constantly overheating.
Good performance and good thermals: it’s going to be a thick laptop (or a desktop) with good airflow, portability will suffer.
Good thermals and good portability: you’ll end up with something like an IPad Pro or a surface. performance will be lackluster.
Good performance and good portability: The thermal solution will be sub par because it’s designed too thin, light, etc. This is basically what we have here with these specced out 16 MacBooks.