How to deal with slow peer reviewing process I submitted a short (10 page long) manuscript to a math journal. It has been 9 months. Almost 4 months ago I emailed them to see what's going on. Their response was: "Your paper has been sent out to review but we haven't heard back".

What are the options at this point? The journal is one of the best in the field but at the same time I'm concerned thaat I might wait a long time just to receive a rejection letter.
As others commented already: Nine month is not unusual in math, and this applies to journals of all levels.

However, 9 month is a good time to send a new friendly reminder to the handling editor. From my experience, this seems to be the time when the editors also get impatient and start to press the assigned reviewers harder (or even proceed with one review less).

I would have written the first friendly reminder after six months (four seem a bit early to me) and a second one after nine month.