Okay, now I'm surprised. I haven't tried to check change in CPU usage, but kernel memory usage does increase. The surprising thing though is that performance increases as well.
The figures below are from apache benchmark. The machine running apache benchmark is maxed out on the 75 byte file test as the test server is the fastest machine currently in the office. In both cases running through vtun is slightly faster.
Without VTun
kmemsize maxheld 2506316
# 75 byte file
Requests per second: 960.12 [#/sec] (mean)
Time per request: 5.208 [ms] (mean)
Time per request: 1.042 [ms] (mean, across all concurrent requests)
Transfer rate: 288.80 [Kbytes/sec] received
# 51,200 byte file
Requests per second: 166.94 [#/sec] (mean)
Time per request: 29.950 [ms] (mean)
Time per request: 5.990 [ms] (mean, across all concurrent requests)
Transfer rate: 8386.21 [Kbytes/sec] received
With VTun
kmemsize maxheld 2626204
# 75 byte file
Requests per second: 987.08 [#/sec] (mean)
Time per request: 5.065 [ms] (mean)
Time per request: 1.013 [ms] (mean, across all concurrent requests)
Transfer rate: 296.91 [Kbytes/sec] received
# 51,200 byte file
Requests per second: 168.39 [#/sec] (mean)
Time per request: 29.693 [ms] (mean)
Time per request: 5.939 [ms] (mean, across all concurrent requests)
Transfer rate: 8459.60 [Kbytes/sec] received
Is there any explanation for this? Lack of state table lookups or the like perhaps?