Andrew, we tested it with ros 2.9.x and it does not work well. The ros machine goes to 100% and we experience ping looses. Did you try with 3.x or 2.9?
Just setup a link from my Soekris net4501 box at home to the office PIX 515E. Pinging 1000 byte packets continuously results in a cpu usage no higher than 18%. Encryption is ESP 3DES.
The net4501 box is based on a 133 Mhz 486 class processor! (MT v3.10)