a 100 mbit switch is only capable of 11-12 MB\s total. If your connection is half duplex (hopefully not!) then you'd end up with about 5 MB\s throughput (since you are reading and writing). Your antivirus / internet security software can also slow things down. So retesting after you install the new switch is probably the right approach.
BTW, for a local DUO copy you can also do a frontview backup job, eliminating the need to transfer all the data to/from the PC.