I'm not sure it's all that CPU intensive in
general. I've been running
OpenVPN to my vps just as an IPV6 tunnel to home for over a year now and it
seldom, in normal use, shows up as more than a couple of percent CPU.
I've just run an IPV4 vs IPV6 speed test from my computer at work
(
http://ipv6-test.com/speedtest/). IPV4 via JANET is giving me
107MBits; IPV6 via my VPS is giving 83.4MBits. During that test,
OpenVPN saturated one CPU on my VPS.
Put another way, in 25 days of uptime, my OpenVPN process has accumulated
78 minutes of CPU time.
So, yes, you could end up CPU-bound, but for "normal" web browsing and
downloads, it's probably a non-issue.