So I don't work at that level, I'd love to hear more. What situations do they leak through in?

you'll mostly notice performance problems if you're not aware of cache lines when writing concurrent software, though it's relevant whenever you read from main memory. As for speculative execution, you probably won't really encounter issues related to that unless you go out of your way to. I don't believe it can actually cause problems in programs (though it is helpful for exfiltrating otherwise secret data).