slow boot (and slow EFI?) in brand new MacBook Pro 16"
I wish I had that fast new gear, but if I were to triage this, I would do some tests with FileVault enabled and FileVault disabled and [avoid the power button. It would be super frustrating to have a slow boot and I’m sure with some tests, there’s a reason why it’s slow and figuring and then solving that technical detail would be satisfying indeed.
As to the process to quantify this startup, use something other than the power button. Use lid open to wake, press any key or connecting power and also use a camera to time things (or narrate while recording to a watch or phone.) Make sure you test no FileVault and automatic log in on a clean erase / install with nothing connected save the Apple power adapter if you suspect hardware being the cause and not software you loaded. Then enable FileVault, and let it finish encrypting and repeat the test. It will pause for you to log in, but you can subtract that delay later - it shifts the boot order on you.
Then once you’re sure the timings are repeatable or know the variance, boot to internet recovery and erase everything and reinstall the latest shipping OS and note that build number once booted. Since you’re on beta, you might have pre-release bridge / firmware and need to wait for that to ship if you want to have Apple look over the logs to see if a repair is in order.

https://support.apple.com/en-us/HT204904

I would not use any install method other than internet recovery and erase install in this case, that ensures you get any bridge, firmware updates slipstreamed into the install process and you do depend on Apple to clean up in the coming months since you hopped on the beta train. If there is something amiss, it should be pretty easy to compare with other similar models and know yours is as fast as them or not. Great work on the 17 sec time - that’s one item to benchmark, but looking at the log files and collecting extra data points might help.