Similar to other posters, I'm in the southern US where temps are normally between 90-100F with humidity 90+%. I'm really slow as it is, but sort of enjoy the heat; however, I do notice a definite increase in AHR and a much slower pace. Oddly enough, it seems to effect me more in the saddle than it does on runs.
Personally, I abhor the dreadmill and make an effort to get outside. If I'm not feeling the heat that day, I'll try to get out earlier or later in the day like others have mentioned. I think the bottom line here is that it's all a matter of individual preference - some like it hot and some like it not.