There are many metrics to measure performance. And an industry ready to supply the hardware to do it. A quick internet search will find advice about how to improve those numbers. There are more, but I do wonder if it's useful. If you're a pro and your livelihood depends on it, then yes. But if you ride at the weekend and maybe do the occasional sportive or charity ride; does it matter?
I initially bought into the hype. I wear a heart rate monitor for most rides. I nearly bought a power meter.
The HRM is used by apps calculate an estimate of how many calories burned. It's useful to know how much food I should eat.
A power meter supplies some numbers that indicate how much force I'm generating. These allow a few things; if the number is big I can boast about it, info about possible training plans and how I might be improving.
Having knowledge about training zones can lead to subscription training services and maybe a coach.
I've no problem with the above if it works for you. I don't think it would work for me.
I have ridden some TT's and hill climbs; with varying success. Which is okay, but I doubt I'd have the time for much structure.
It could be motivational, but I've not had much of a problem with that. Exercise has become part of my routine.
I was a participant in some research in Jan 2022. This meant I tested my Vo2 max. I don't know what it is now; it was 51. A lot has happened since so I am thinking about having it tested again. Does it matter? Not really. I doubt I'd change the amount of riding I do.
Another popular test is lactate threshold. A measure of how well your body deals with and tolorates the byproduct of working muscles. Obviously I have no idea what my score is.
The more I think about the tech and the tests, the less likely I am to buy into them. I feel I know my body well enough to gauge how it will react to the load I might place on it. I know when I can push harder and when to ease up. And how far is too much.
Saturday club rides are another way to measure changes. Especially if it attracts new, younger people to keep up with.
Lastly the bikepacking trips I started last year place extra strain on me. The multi day makeup of them demonstrates how well I can recover.