How to find out the average duration of the peer-review process for a given journal? I have a mathematics paper ready for publishing, but I can't decide to which journal should I publish it.

The main thing based on which I want to publish the paper to a specific journal is its short period of peer-review process (because I need to publish it as soon as possible).

Suppose I have a list of journals I want to publish the paper. How should I know the average period of peer-review process for each of them?
For publishing in mathematics you could check out the AMS' Backlog of Mathematics
Research Journals. Among other things it provides a current estimate of
waiting time between submission and publication and historical data for the time between submission and final acceptance.

I'd personally take these estimates with a grain of salt, but I suppose if you're desperate it's better than nothing.