Write performance looks like saw

Running Rockstor on N3700 with single WD RED drive. When reading file from user’s home folder I’m getting 300 MBit/s. But writing bigger files is a pain. It writes a block, maybe 100 MB, then freeze for 5 seconds, then next block. Graph is below.

CPU usage on dashboard is 100% all the time. ‘top’ shows either smbd or mnt-share or some others loading cpu up to full.

What might be the problem?