How to deal with slow peer reviewing process
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.