Heat wave slowing my swimming?

... but not my running today. Go figure. Today, I did a swim workout on my own, not a lot of yards, abt. 2k, and I felt sluggish. Times (slow by most standards on a good day) were even slower today. We've had a heat wave the last few days in the Philly area, and I figure the heat may have contributed to the slowed down times. But then, following the swim workout, I did a running interval workout and that went really well w/ my times just where I wanted them! Strange b/c I was outdoors so it was a good deal warmer than in the pool (where the water tends toward warmish, maybe a little too warm given the weather). I seemed to find the energy that wasn't there in my swim. Same heat wave, same body.... True, running is more familiar territory than swimming for me. But interesting how I would have perked up during the running workout. What's your experience with swimming during heat waves? I must say that even a too warm pool is a welcome place when Philly style heat waves descend, but maybe not conducive to performance.