Four years ago the heat had a big effect on the 200, a much shorter race. Maurice Green and Michael Johnson both cramped up and dnf'ed. It's the cumulative effect of multiple days of competition, warm ups & warmdowns, not being in normal eating and drinking routines. Definitely effects the 1500, especially with multiple rounds.
Have to agree with Chuck D on this one. 4 minutes is too short to see a big impact on performance.
As a heat transfer professional (when I'm not being a nut doctor), I can tell you that statements like "it was 110 degrees on the track" mean absolutely nothing. Your comfort level is affected by a bunch of things such as: temperature, humidity, sunshine, temperature of the surface you are standing on, wind, and even the haze level. When they ran the race the air temperature was 95F (pretty hot), the dewpoint was 55F (not particularly humid), there was moderate sunshine at 5pm, but the track itself was probably fairly warm cuz it's been out in the sun all day.
To counteract this nasty heat, the body dumps heat three ways: (1) sweating which leads to evaporation from the skin (2) *breathing*, because you take in relatively cool and dry air and it leaves your body warmer and more humid and (3) circulating blood close to your skin.
The biggest thing that kills you in the heat is dehydration and 4 minutes is way too short to get dehydrated. It is true that you will sweat more and circulate more blood and breathe just a bit harder, but everything I've observed is that it is all small potatoes, particularly for an athlete who is acclimated.
Like Chuck said, you didn't hear many complaints, did you? Didn't several runners get near their season's best time? Webb didn't, but that's because he didn't have a rabbit, the pacing was strange and nobody was pushing him.
It is an interesting question. What is the shorest duration of race that is affected by the heat? I agree that there do not seem to be many, if any, studies on races this short so it is tough to say. I think there might be 2 avenues through which performacnce could be affected in a race as short as a 1500m. If a runner's core temperature gets up to the 39-40 degree C range he/she will not be able to sustain the same intensity of exercise. If this would occur somewhere in the first hhalf of a 1500m I could see how it would slow a runner over the second half. Much would depend on the core temp. at which a runner stared the race. Secondly, there has been one published report that high ambient temperature is associated with a lower intensity at which the lactate threshold is reached. Here is an abstract. Didn't get a chance to track down the entire thing.
LACTATE THRESHOLD IS AFFECTED MORE BY HEAT THAN BY HYDRATION
Papadopoulos, C., Doyle, J. A., & Thompson, W. R. (1998). The effect of hypohydration on the lactate threshold in a hot and humid environment. Medicine and Science in Sports and Exercise, 30(5), Supplement abstract 1612.
The effect of hypohydration on lactate threshold in a hot and humid environment was evaluated. Males (N = 10) performed four randomly assigned maximal treadmill tests. Two trials were at room temperature (22 degrees Celsius) and two were in a heat chamber (37 degrees Celsius). One trial was euhyrdated and the other hypohydrated. Hypohydration was restricted to -1 to -2.6% change in body weight.
Lactate threshold occurred at a significantly earlier stage, lower VO2, and higher core temperature in the heat trials than in the room temperature trials. Lactate threshold was not affected by hydration.
Implication. Temperature has a greater effect upon lactate threshold than hydration.