combine SSD and HD

If you did a factory reset, you could create a 1.5 TB RAID-5 array with all 4 disks. I don't recommend it though - you won't see much performance gain, since sequential reads/writes still cut across all four disks.

Your second idea is a better approach - switch to flex-raid, and create additional volumes. RAID-1 for the SSD's is possible; there are two other options you could consider. If you have a good backup for the SSD array you could use spanning RAID-0 (creating 1 512 GB volume using both disks). I don't usually recommend this, since if either SSD fails you lose everything in the volume. But if you know that up front and plan for it, it is viable. You could also create two new volumes, 1 for each SSD.

You likely won't see much (if any) improvement in streaming performance. You should see faster access to small files and and the database itself (since there is no seek time, random i/o is faster).

Thanks. I guess I'll try the FlexRAID then. Ja, it is a bit odd, but I really have problems getting good streaming from the NAS on WLAN, even if the connection appears to be healthy (e.g. sitting nearby). This is especially for TV Shows, but sometimes music, too. I was thinking this could be partly remedied by faster read (I/O) from the discs.

P

More than likely the issue is actually with your WiFi - perhaps interference?

Perhaps test the speed with NasTester. http://www.808.dk/?code-csharp-nas-performance

Hmmm, ja, I am not sure about that. I mean I considered Wifi issues and changed channels around manually (because the FritzBox reliably picks a channel that has interference), but that did not help much. Also when streaming directly from the Web (e.g. Amazon Prime. live streams) there are no such issues. Also even when connecting to the router over Ethernet from my Mac music streaming is not really faster (NAS is connected via ethernet, too).
Thanks for the link to the tool, I'll check that out.

P