The problem with your testing regime is that the mysterious slowdown may only happen every several weeks and then not always for the same device.
While it can happen at any time of the day of night, I most often get this late-ish, around 11:30 pm - midnight. I looked at the backup schedule and saw it was set to 2:15 a.m. So, that shouldn't be cause, unless how it calculates the time zone is off somehow but per this screenshot, everything looks right:
The only thing I'll mention about the settings is that I just put in my zip code the other day, but the time zone has been set to US/Eastern forever (and I live in the Orlando, FL area).
In any event, I changed it to 3:30 a.m. and every other day.
