Is the question asked because you just want to know if your avg speed is going up? (probably not)

Are you asking this question because you want to know if your physical conditioning is better? (most likely)

I'm no physiologist but I think you need to look at your resting heart rate, your elevated heart rate for some fixed duration and how fast your heart rate drop back down. For instance, if you ride hard and come to a stop at a light/stop sign, does your heart rate drop down quickly or does it take a long time for you to recover. Panting longer time or shorter time.

Speed is one thing but your heart rate, elevated heart rate, how fast you recover is more of an indication to your conditioning.

Some are naturally gifted at running, or swimming, or bike riding. Others are not. It's not that you are out of shape but rather your physique isn't built for the sport. I'm small with short legs. Can't possibly run against someone with European or African ancestry. For each step they take I need to take more than one...

Pay attention to your heart rate, how fast it responds to physical stress and how fast it recovers.