I have a full backup from 2017, when was the first time the black bytes were created/distributed?
On 25 December 2016.
However I don't know when you started participating in distributions.
If you created the backup later than you first received blackbytes, they should be there.
The bottleneck is usually in CPU or disk speed, not in network connection.
You don't need to be connected to witnesses. In fact none of the witnesses accepts connections.
The sync takes about one day per day, Obyte runs on an i7 of the 7th generation with a CPU utilisation of about 10%.
That's terribly slow. Do you have a SSD or a magnetic HDD on this computer?