Yeah, I think anyone who's been to Rio, or Jeddah, on a sunny summer day knows that the heat impairs a lot as well.

The average high of the hottest month in Rio is only slightly warmer than the average high of the hottest month in Boston. So it seems that summer heat should not be much more of a problem in typical tropical climates than in temperate high-latitude cities like Boston.

Overall the net effect of high-latitude environment (minus tropical disease protection) is probably very negative, with substantial efficiency losses relative to tropical type climates. The benefits of avoiding tropical disease would more than compensate, however.