could be a number of things, you just have to start from basics, what speed are you getting at the moment?
how are you connecting NAS, cable to your router or to pc ? or wifi. where are you transferring the files to, from NAS to PC
if connected to your router by Ethernet cable, first put it direct to your pc by Ethernet cable and check you can get a good speed there first. Could be your router slowing the speed, check port speeds on router. ideally you need Gigabit ports for full speed
if by wifi what band you using 2.4 or 5 GHZ see if you can connect using the 5GHZ as its faster with new devices, also try a different channel. You can get a phone app to scan and see what devices are using what channel so you can use a channel that's free.
I had a sinology and if I remember right when I had one you have to change or check that your running file transfer in SMB3 protocol
a quick google on what I had originally came up with the issue and fix on my unit