Do we have to choose one or the other? This seems like a false dichotomy. How about no malware/badware and optimal heat dissipation? That's the route I go. I've yet to acquire malware from a can of compressed air and a Phillips screwdriver.

And the performance impact can be absolutely extraordinary. I've seen systems go from barely functional to completely decent via a simply cleaning. Note that there were no faults or errors in the former case, it simply was forced into perpetual thermal throttling. In modern processors this case halve and worse your performance with ease.