Is there any differences in what will be backed up between an encrypted and unencrypted Time Machine Backup?
The restore time is not significantly different in my case between the two backups.
My old Synology was actually the bottleneck when compared to local USB 2 and USB 3 drives - LAN use was around 70% of max on the console (80%+ of max LAN speed is about the best I could ever get out of the Synology and I think that was fluke).
If speed is key then look to use something like Carbon Copy - I find both backups and restores faster with this than TimeMachine.