Here is my data:
- low-spec linux machine (4 cores, 8GB ram, SSD+HDD hybrid disk) with 12 folders 348 GB on disk and 180k files took 102 seconds to complete
I must say this is very much the normal startup time for this machine. It was almost impossible to notice that any migration was going on..
EDIT: maxFolderConcurrency is set to 2 on this machine, in case this is relevant
EDIT2: DB size shrunk by ~20% from 1.0 to 0.8 GB
EDIT3: Another update on a machine with the same specs as above but even slower HDD took 152 seconds for 13 folders with ~205k files and 800GB on disk. DB size lowered from 1.2GB to a bit below 1 GB.
But the real thing is the startup time after login. This system took 13/14 minutes before every folder was synced on 2.0.8. Now, it takes only 6 minutes with 2.0.9! Wow 
I’ve tested this twice even with a reboot in between to avoid side effects but the result is the same. Thank you very much for this significant improvement!
Edit4: 4 devices in total