Today I’m doing a bit of cleanup and new backups on the NAS after my recent upgrade  of the firmware and I’ve found the speed to have decreased (almost halved) in comparison to DSM4.3 when transferring large files (about 8GB in a 9 files)

The web interface shows a very high CPU usage, but no errors.

Going through the ssh interface top shows that the SMB demon is responsible for this:
Mem: 228728K used, 20560K free, 0K shrd, 11452K buff, 157692K cached
CPU: 6.1% usr 21.9% sys 0.0% nic 0.9% idle 1.1% io 0.3% irq 69.3% sirq
Load average: 3.08 1.94 1.12 2/178 31877
PID PPID USER STAT VSZ %MEM %CPU COMMAND
12426 6493 root R 31948 12.7 88.9 /usr/syno/sbin/smbd -F
31028 2 root SW 0 0.0 5.7 [flush-253:3]
128 2 root SW 0 0.0 1.1 [kswapd0]
2903 2 root SW 0 0.0 0.9 [md2_raid1]
31390 31387 admin R 4348 1.7 0.4 top

In non-synology-related posts around the web it looks like the issue could be tied to the very high CPU consumed for sirq.

I hope that a new firmware will bring back the old performance, in the meanwhile I suggest to other owners of this NAS to refrain from upgrading.