Adobe Firefly use in generative fill

If we buy a Firefly paid plan, can we use its credits in Generative Fill Photoshop with that account?

Yes as long as you also have a version of Photoshop which has generative AI features (version 25 or later, I believe). The FAQ document (document link listed below) has a table which lists the applications that use the generative credits.

And on the Adobe website, it says that if we run out of credits, the speed of generating will be a little slower, that is, how slow is it and how long or how many minutes does it take?

Adobe has not answered that question in detail.  The information in the generative credits FAQ is the latest information and they update that page as things change. It is located at:

Frequently asked questions about generative credits