Time Machine seems like ripe for some kind of overhaul. "The first backup is slow" is not a new bug in 10.15.3. I had that issue in at least 10.14 as well, and

Time Machine seems like ripe for some kind of overhaul. "The first backup is slow" is not a new bug in 10.15.3. I had that issue in at least 10.14 as well, and who knows which other versions. Time Machine GUI is great; but for backups off the device you need NetAtalk (which has been reverse engineered though). The TM GUI would've been great with ZFS back in the days, if that deal went through. It did not. APFS does not support deduplication. There's a bunch of Time Machine GUI FOSS applications which are inspired by Time Machine GUI, for Linux. Not sure which one works best, or if they work on macOS.