Time Machine backups are too slow
Time Machine backups are limited by three factors, the type of scan selected, the speed of the primary storage, the speed of the destination storage.

If your empirical evidence is increasing destination, then that’s not your bottleneck. (Sorry to restate the obvious)

I would look over the unified log file to see what’s actually happening and also not mess with any defaults (reset them if you changed anything) and use this tool to get some specific data:


https://eclecticlight.co/2019/12/27/t2m2-further-improves-strategy-data-for-backups-in-catalina/


It is a nice graphical wrapper and the author explains very well in their writing how it works, what some gotchas are and even how to read the system logs if you find you need different data than the Time Machine Mechanic utility exposes.

In the end, it’s probably more likely you need to exclude whatever data is taking a long time to scan or just live with the backup speeds, but I’d love a follow up question with some specifics if we want to get into if it’s bandwidth or iops that’s constraining your backups.


Time Machine Backups taking too much space?


Also, case sensitive is an odd choice and not usual - do you have room to make a second partition and test the defaults journaled / encrypted HFS destination? The encryption isn’t going to make a difference in my experience either - so that’s a good thing to not test unless you’ve exhausted every other variable.