I'm fascinated by problems, that are NP complete, but still can have a ton of optimizations that can reduce the time needed by many orders of magnitude (million

I'm fascinated by problems, that are NP complete, but still can have a ton of optimizations that can reduce the time needed by many orders of magnitude (millions or more), like SAT solving. Or approximate solutions / heuristics like approx. traveling salesman. Seeing huge improvements on real-life problems, even if you know the problem is asymptotically exponential feels like you've fought against impossible odds and won.