Through this link, https://www.elsevier.com/physical-sciences-and-engineering/mathematics/journals/fast-publication, one sees the publishing times for selected Elsevier journals, given by Elsevier itself. My question is whether there are averaging times listed for more/all of Elsevier journals, and if so, where to find them. (for example, the Journal of Number Theory)
At least for mathematics journals, the AMS has a list of average waiting times:
You can try this: https://journalfinder.elsevier.com. When you add your paper title and abstract, it gives a list of journals, which includes the 'Review speed' as well. I assume with you paper title and paper abstract, the journal that you're looking for should be in the suggested list. Here is the snapshot of one example, in which I put a simple title and abstract for a paper. You can see the results at the bottom of the page and I have put the 'review speed' in the red rectangles:
I hope it helps.