What's the hardest (longest-running) financial calculation?

[HP67]

I'm not sure what you meant but in terms of processor speed I think I read the 30B is one of the fastest calculators ever. Still, I find the old classics faster to use than the newer models. HP guys were masters of UI. For me the 48 is faster to use than the 50 and the 12C is faster than the 30B even though the new models have much faster processors, just because of how well laid out the most used functions and menus were on the older ones.