Tried running ZS 3.5 on a Compaq CQ2302F machine with on-board Realtek looking into the LAN and USB attached Linksys EtherFast USB100TX card looking into the DSL modem.
Initially the d/l speed was on par with the DSL connection, but quickly degraded to only 100 KB/s. As I do not have a x1 PCIe low profile card to test with this machine, had to return the old ZS 3.0 router into service. So I am not sure if the root cause was ZS 3.5 running on this puny Atom machine, or the USB adapter itself.