I have completely rewritten this comment, so Reed Crosby's comment may not be exactly coherent with my comment.
I found what I think is an/the early note on reddit about this at https://www.reddit.com/r/iphone/comments.... So, although we don't know what exactly triggers the throttling (battery cycles or capacity or age or ?), we do know a way with either CPU Dasher X or comparative Geekbench scores to see if you are being throttled.
I did a cursory security check on CPU Dasher X (no big red flags when searching the web, it looks like there is open source code on github for some version of it), downloaded it, and found that I am not throttled (CPU speed very close to max). The coconutBattery application (runs on your computer but will check you phone's battery when connected) says my phone's battery is still above 95% / 1545 mAh at only 257 cycles, even though the phone is 15 months (about 450 days) old. I would guess it is capacity or cycles based, as it seems like I might get throttled on age alone. I'll comment back here if I notice a change and remember to come back.