This has little to do with bloat. There's a complex array of backend systems involved in logging you in to your bank: Authentication, authorization, customer system, separate account systems for each account type (debit, credit, loans, investments), etc. The bank's login process has to wait for all of them to respond and collate the data before returning control to the UI.
Rest assured, online banking teams spend a lot of time trying to optimize login performance but they're limited by the slowest of the downstream systems. There's a lot of data on that "account summary" page you see after logging in.
The login process performance could be improved by landing on a simple "welcome" page but the majority of people want to see all of their account balances immediately upon login.
Source: I have worked on several online bank websites and mobile apps.