I have submitted the paper to an Elsevier journal and one of the reviewers gave his review in just 5 days. However the paper still appears to be under review. I have 2 questions:
Your question is based on a wrong assumption, namely that fast reviews mean reject. I myself have accepted reviews and finished the review a day later with an accept recommendation. This was because I happened to have time that day. As a reviewer, you cannot spend an inordinate amount of time on a review. In Computer Science, if I am familiar with the subject, I can usually finish a review in a few hours. I usually wait for a night and a final second look before I submit a review. If I am not that familiar with the area, then of course it takes longer. I am not holding myself up as a good example, but just to assure you that a positive review might be done within five days based on my own experience.
If your field is Mathematics and you write in the typical terse style, then reviews are just bound to take longer, because reading Mathematics papers involves much more work. But then, when I was still a working Mathematician, the (bitter and cynical) joke around my colleagues was that the average number of readers of a Mathematics paper was less than 1.0, including reviewers and editors. Even in this case, if the review request arrives to the reviewer who has time (or wants to procrastinate grading, ...) the reviewer might do all the work within a few days.
Finally, you also assume that a single negative review will sink your paper. Even at a very prestigious journal, where one negative review will usually lead to a reject, the editor reads the reviews, reads the self-assessment and comments to the editor (such as, "I think this is non-sense but I have not done Beginning of Life Biology for decades and am now working in Experimental Archeology of South New Zealand, maybe you confused me with my cousin"), and only after looking at all the input from the reviewers will the editor make a decision.
If the first review was negative and the editor is going to reject on that basis, then I agree that they should do so as soon as possible. This not only saves your time but will save the other reviewer (who probably hasn't even started their review yet) some work.
However, there are several other possibilities.
The first review was not actually negative. It sometimes happens that a review request arrives when I have no other active requests, I have some time available for reviewing, and the paper is easy to read. A quick positive review could happen in those circumstances.
The editor hasn't had time to look at the first review yet. Editors have busy and less-busy times too.
The editor has looked at the review, and although it was negative it was not very convincing (perhaps the reviewer does not seem to have read the paper properly), so they intend to ignore it if the other review is thorough and positive.
It's not possible to determine what the recommendation from a reviewer is just based on the number of days a reviewer spent on the paper.
This is not a question, but your opinion. But an editor might still ask for revisions or accept the paper even though one reviewer recommends rejection.
