Another more general property that might be worth looking for would be substantially similar posts that get cross-posted to a wide variety of communities in a short period of time. That's a pattern that can have legitimate reasons but it's probably worth raising a flag to draw extra scrutiny.
One idea for making it computationally lightweight but also robust against bots "tweaking" the wording of each post might be to fingerprint each post based on rare word usage. Spam is likely to mention the brand name of whatever product it's hawking, which is probably not going to be a commonly used word. So if a bunch of posts come along that all use the same rare words all at once, that's suspicious. I could also easily see situations where this gives false positives, of course - if some product suddenly does something newsworthy you could see a spew of legitimate posts about it in a variety of communities. But no automated spam checker is perfect.