The 90 second timeout is for how long SB will wait for the page to load before skipping onto the next.
The comment setting for the delay of 1-10 secs or RND is how long SB waits to start trying load the next page to post a comment on. This delay makes it look a little more realistic if you happen to be posting comments on multiple pages of the same blog.
If you comment on 20 pages and the time spent on each page is exactly the same, it's kinda obvious you are a spammer. The site admin can't tell how long it took to load the page, but he can see how long you spent on the page after it loaded.