I'm very happy to report that by rearranging the order of network operations slightly, I have been able to bring the plugins' CPU usage down to essentially zero.
The original NetVST implementation was pretty good, but CPU usage of the plugins (especially on the Mac) was stubbornly high, around 15%. At that rate, it didn't seem practical to use many plugin instances. With the newest implementation, this issue is gone.
Head to the wiki downloads page to get the updated NetVST and plugins. Note you must update both the NetVST Windows executable and all NetVST plugins, because this change breaks backward compatibility.