I might know the answer to your last point. I have experience working with financial services and large old institutions.
In short the front end is likely lighting fast and lightweight but the services it relies are incredibly old and outdated. Like mainframes running COBOL old. There is likely some abstraction but there are also likely literal decades of technical debt. Sometimes a call to understand what should be simple like what accounts does this client have might need to call multiple legacy systems that were integrated over the course of multiple acquisitions.
I might know the answer to your last point. I have experience working with financial services and large old institutions.
In short the front end is likely lighting fast and lightweight but the services it relies are incredibly old and outdated. Like mainframes running COBOL old. There is likely some abstraction but there are also likely literal decades of technical debt. Sometimes a call to understand what should be simple like what accounts does this client have might need to call multiple legacy systems that were integrated over the course of multiple acquisitions.