Performance optimizations for high-bandwidth Tor exit

[6cnf6cp02 at sneakemail.com]

Hallo,

I've been running a Tor exit node on one of my machines (Intel Dual 
E2160 (1.8GHz), 2GB RAM, Xen domU for Tor, encrypted HDD) for some 
months. It is on a shared 100MBit/s line (500GB in/out daily). I 
have not configured any bandwidth limits within Tor.

Most of the time, the Tor process maxes out the CPU (85-100%), 
while memory consumption stays at ~10%; until today, this didn't 
pose much of a problem as log files show no errors and the machine
has been used exclusively for Tor.

I now want to play around with hidden services, and noticed that
Apache takes a very long time to reply, even to local requests.

As I am already using Xen, I thought it might be possible to
share CPU and memory intelligently between two domUs (with the
Tor domU having lower priority), but I didn't find any useful
information how to do that.
Are there any performance tweaks to limit Tor's CPU consumption? 
NumCPUs is set to 2, other than that I didn't modify the default 
configuration much. Also, niceness is set to +15 for the tor
process. I want to avoid setting a fixed bandwidth limit, Tor 
should use "as much as it gets". Will upgrading the CPU help?

-Daniel