NAS326 Transmission Low Download Speed

Well, as you already guessed, your desktop pc has a lot more oomph, unless it is produced in the previous millennium.

I don't think the memory will be a problem, but either the CPU or the disk(s). When you have ordinary disks, this will be the bottleneck. A torrent comes in in chunks of 512kB or 1024kb, which are randomly located in the file-to-be-received. Which means that at 30MB/sec the head has to be moved somewhere around 60 times a second. With a typical random access time of 12 msec, this means the heads are moving 720msec per second, which leaves 280msec to actually write that data. 30MB in 280msec equals 107MB/sec, which is about what you can expect from a mechanical disk.

When your desktop does a better job, I guess it has an SSD.

Raid0 can give a theoretical improvement, but in practice I doubt if you will find much difference. As your bandwidth is mainly crippled by the seektimes, the chunk size of your raid0 array should have to be a multitude of the torrent chunk size, and by aligned. In other cases the number of sequential seeks won't be less.

A SSD might give a boost, but if you want to go that way, first check if the CPU has some cycles left when downloading at 30MB/sec. There is a page in the webinterface where you can see the CPU load. When the CPU is above 60%, don't expect to be able to get 50MB/sec.