One possible scenario where this delay could be because of a security measure is if the site is using something like a Client Puzzle Protocol (CPP). This does not appear to the case with https://www.chase.com/, but CPPs can be used prevent denial of service attacks against slow hashing functions. It is basically an implementation of Proof-of-Work system. More details here.
The basic idea is to force the client to do a significant amount of work, and prove it has done so, before you will accept a username/password pair and try to validate it. Basic overview of the approach from the linked post:
The server generates a random puzzle, and sends the puzzle to the
client. The server generates the puzzle in a way that it can predict
reasonably well how much effort will be required to solve the puzzle
(e.g., 100ms of computation). The client solves the puzzle, and then
sends the solution along with the user's username and password.
and the puzzle description. A legitimate user's web browser would run
the solution in the form along with the username and password.
Based on how this is implemented, a site could delay loading of the login page until the client (your browser) solves the puzzle. Again a bad UI design -- I'd just disable the login button until the puzzle is solved and enable the button for login form submission once the client has a solution.