Does a bad paper get rejected very fast and if a paper is under review for long time, should one expect at least reasonable review? My question is regarding applied Math journals (for example SIAM). Is there such a thing that a bad paper gets rejected very fast? Also if a paper is under review for more than 6 months, then should one expect at least a reasonable review (Although it may get rejected at the end)?
No.  There are tremendous differences across journals.  A "bad" paper can get rejected within hours after submission, or 6 months after submission.  Good papers can also get rejected within the same timeframe.  Some reviewers are very conscientious and complete the reviews quickly, but the paper can still sit with the editor for a long time.  Or, more often than not, you have multiple reviewers on much different time schedules.  Some journals do a very good job of triaging papers, so a rejection of a bad paper may happen quickly.  But you will learn that "bad" is a very subjective viewpoint.  I suggest that you do not associate the quality of the paper (i.e., good vs. bad) with the time under review.