Time for journal reviewing / referring in mathematics - Is there a recent study?
The American Mathematical Society provides regular (annual, I believe) on journal backlog times. The latest can be found here: https://www.ams.org/journals/notices/201910/rnoti-p1713.pdf
Depending on your subfield it may not contain data on all relevant journals, but it is quite comprehensive. The reports are published in the November issue of the Notices of the AMS, which is also available online.

The data in the report is based on information provided by the journals itself.