The first looks like it fails to acquire a lock in order to gather storage statistics. I’m not sure if this slows down your cron or not.
This indicates that you are doing a lot of writes (my guess, not 100% sure ), and the checkpoint takes a longer time to finish. Check Internals - Checkpoints and Journaling - #3 by Alexandre_Araujo for more info.
As you have more merchants, i believe it will be difficult for your cron to catch up. (e.g. may run even longer than 15mins internal).