Without details, I have big and strange problem.
On 1Gb local network, I have ~100 MB/s speed when trasfer files:
PC (win)<>NAS1/2 (smb/cifs) - network places
NAS1<>NAS2 via web browser (http)
Great! And that show that network,switch,cables,smb... are OK
But (which I use a lot, every day, and...