Pentium D processors are pretty power hungry, so factor that into your thoughts. Also make sure you put a modern OS on it that is getting security updates. It probably has Win XP or Vista installed which isn’t safe to connect to any network.
It should work fine as a router as long as you don’t enable any of the packet inspection features. For basic routing and firewalling for a home network it should be plenty powerful. I would personally put a small SATA SSD in it as the main drive and ditch the 90GB HDD.
As an additional idea, if you put a larger SATA drive or two into it you could make it a NAS.
The biggest thing is you have changed a random write to a linear write, something HDDs are significantly better at. The torrent is downloading little pieces from all over the place, requiring the HDD to move it’s head all over the place to write them. But when simply copying off the ssd, it keeps the head in roughly one place and just writes lineally, utilizing it’s maximum write speed.
I would say try it out, see if it helps.
Also, if the HDD is having to do other tasks at the same time, that will slow it down as the head can only ever be in one place.