n.b. I think this answer is more relevant outside of mathematics, which is the OP's field ...
Although @Thomas's answer is correct that most journals to which you could submit your paper will be sharing the same reviewer pool, there can be big differences between journals in how efficiently their editorial processes run and, importantly, how much they pressure authors to return reviews quickly.
- You can look on a journal's web page to see if they give any metrics, or emphasize speed: for example, Nature Communications says in their Aims and Scope section:
We are committed to providing an efficient service for both authors and readers. Our team of independent editors make rapid and fair publication decisions.
Obviously that doesn't give you anything hard and fast, but it does at least tell you that they prioritize speed.
The other point I made in my previous answer about rapid peer review is that depending on your situation, it might not be as important as you think to have your paper published; in many cases, submission to a reputable journal counts for almost as much as publication - it indicates to potential admissions committees, employers, etc. that your work is actually ready for prime time (as opposed to "in prep", which can mean anything from "I've got a good idea" to "submitting tomorrow"). Depending on journal policies etc. within your field, you could also consider posting your paper to a pre-print service such as ArXiv - another way of convincing people that your work is for real.
The best way to figure out the true importance of rapid publication for your situation is probably to talk to a senior colleague in your field who knows your situation.