Hi Peter,
We use a VPNc 3030 and terminate many different devices on it inc, pix, asa, routers etc. We havent noticed any problems with throughput over the tunnels.
We use an older SEP module and it handles 3DES encryption fine. You could check to make sure that the SEP module/s are enabled for your tunnels and that there is no bandwidth policies or default bandwidth policies in place??
The 3030 is a VPN platform designed for medium- to large-sized organizations with bandwidth requirements from full T1/E1 through T3/E3 (50 Mbps maximum performance) and up to 1500 simultaneous sessions.
Hope this helps.
Rgds,
Ian.