I started riding regularly at the end of March. My speed has been gradually improving since then. Tuesday I went on a ride, and my average speed was 17.5mph, and my average heart rate was 157bpm. I had intended to take it a little easier on Wednesday, but maybe not as easy as I should have. My average speed was 16.8 and my heart rate 150. Today, after 2 days rest, my average speed was 16.4 and my heart rate was 172. This is all over the same section of road. At the end of the ride it felt like I had goose bumps, but it was 80 degrees outside.

I started tracking my calories on Sunday, and at the same time started a calorie deficit. I am not exactly sure what my consumption was previously, but I reduced it to 1600 per day based on the recommendation from myfitnesspal.com. I chose to lose 1 lb per week. So far I have been sticking to it pretty well, and I did go over a couple hundred on the days I exercised. Could my reduced performance today be related to my reduced calorie consumption? For the record, I think I had a protein smoothy for breakfast the first two rides, and a bowl of oatmeal and an energy bar this morning.

Being dehydrated or overheated will cause your HR to be higher for the same power. Goosebumps when it's hot are a sign of being overheated or dehydrated, I forget which.

Calorie formulas are often wildly inaccurate, usually high.

It could. I have used Myfitnesspal, and it has you add back your exercise calories to your base calories for the day to keep you at a steady weight loss. You don't say how far you rode, but I am pretty sure you burned more than a couple hundred calories. If that is the case, you were at a steep calorie deficit and did not have enough energy to fuel your ride.

I rode 20 miles that day.

I did the same ride again on Monday. Averaged 17.6mph and heart rate was normal for the effort I put in. The fact that my performance was down for one day isn't a big deal. I am just trying to understand why, so I can use that knowledge to make better choices in the future.

Average speed is not an accurate measure of performance. Neither is heart rate.