Assuming they knock and announce they have a warrant as opposed to knocking the door down, you open the door and they start coming in immediately. You can read the warrant, they'll likely shove it in your face. You should be on the phone to an attorney or the local bar association before the last officer in past the threshold, anyway.
I'm pretty shocked an attorney would suggest doing something that can very likely be construed as stalling for time when tensions are likely pretty high.
Nor do they have to allow you time to call a "watch commander to verify it".
What is likely, however, is that you're gonna be shopping for a new front door if you don't open up by about the second knock -- if they give you that long.
Obviously this isn't legal advice, but it's what defense lawyer friends have suggested if it ever comes up.
I think that popular video about "never talk to cops" mentions something like it too?
I assume if they don't wait, it helps your court case. They could always go with the "probable cause" argument, but again this can help your case if you can prove they didn't have actual cause.
Pro-tip: if they're at your door with a warrant, they have probable cause.
https://www.motherjones.com/politics/2010/09/aiyana-stanley-...
Think about it: say someone has a bunch of drugs, you wouldn't want to give them time to start flushing it down the toilet while they call up and verify the warrant is valid. You kick the door in, arrest them, and secure the scene for forensics and evidence collection.