If we talk about that if someone posts too quickly than he might be spamming the forum with low quality posts, it looks reasonable.
Mostly if a poster makes some posts within a short time, it can be spam posts which are more likely low quality. However and fortunately, it is not always true.
Another poster can open multiple topic, click on Reply, open multiple reply box and compose his replies. When he finish one post, he can spend sometime to rethink about the idea, while composing other posts. Later he can come back and finalize his replies, click on post to release some posts within few seconds. I don't think with this poster, it should be considered as spam or burst posting.
Quality is most important to consider, not how long or short time between your posts. You can wait 2 hours between your 2 posts but they are spam because they are low quality.
You're all right about this, no matter how fast a person can type on a key board, at least it is expected of such person to first read through a post and understand it before giving a reply or comment on that thread, which on an average takes nothing less than 3 minutes to do so but when you discover a user making about two to three posts within this three minutes then it calls for a suspicious activity ongoing, you need time to also go through threads before seeing your desired topic to post under, we can also use the content of the post one made in deciding a spammer or not.