In my youth, the 1960s, there was a saying for Savings & Loans [what Brits call Building Societies]: pay depositors 3%, lend at 6%, and hit the golf course at 3pm.
The spread was fairly high, but it wasn’t high enough to keep the S&Ls in business when they had to pay depositors more because of Vietnam War induced inflation. The S&Ls tried to save themselves buy lending on office buildings, but that flopped.
Shucks the real money is made by lending money they don't even have.