They are Nigerian, so they have terrible writing skills. I mean, bad. Yes, not everyone can write like they're an English professor, but these guys are awful, and it's not because they're writing quickly, it's because they don't know the English language.
Here's an example of a scammer emailing me on a dating site:
The guy can't even write lol properly!
Oh... and he can't even get his name straight. On his profile, his name was Barry, but when he emailed me, his name was John, so now he says his name is Barry John.