You could have had an incompatibility between the Synology Ethernet ports and the router switch ports that caused the NICs to operate only at 100 Mbps instead of Gigabit.
Again, the routing portion of a router isn't involved in LAN to LAN transfers.
You could try the Ubiquiti. It might help.
Again, the routing portion of a router isn't involved in LAN to LAN transfers.
You could try the Ubiquiti. It might help.