[marcmac]

This is on M4 (41, I assume?)

I've run similar tests, and your system load shouldn't be that high. I'm not sure how much this is related to architecture, but some things to further investigate:

When you're injecting the messages, how large do the postfix queues get? How many lmtp processes (in postfix) seem busy? Are the messages backing up before amavis, or after? Try tailing /opt/zimbra/log/zimbra.log and look for the message deliveries - are they coming fast, or slow?

Is /opt/zimbra/amavisd-new-2.3.3/tmp mounted as tmpfs? That speeds up amavis quite a bit.

What's the IOWAIT on the box like? What kind of disk are you using? (local, san, nfs, usb drive...)

This isn't running in some kind of virtual machine, is it?

What's in the process list other than zimbra?