No, I did some deep testing, USB virtualization doesn’t seem to be limiting performance/bandwidth in any way.
If you’re not using the latest 2.0 ZS release, try upgrading. The problem seems to be gone for me since I have switched to rev 2.0 (although I need to test it further).
A VMWare virtual machine is not yet available for 2.0, so you’ll have to build it your own (not a difficult task anyway).
How much better?
I’m having the same problem and can’t solve it. But I can add something that seems relevant to me: no matter what, download speed always reaches 1,5 Mbps and upload always 750 Kbps. Never more that that, and rarely slightly below those thresholds.
Just for the sake of completeness, using Vodafone Italy on a Huawei K3765. The same combo, when connected directly to the same laptop I’m testing ZS on (using VMWare Player), from the same place, will do about 5 Mbps/3 Mbps (download/upload).