I have the data sheet for the 506E and as per the data sheet, a clear text throughput of 100 Mbps is supported. The throughput is pretty high even with encryption (56-bit DES IPsec VPN throughput: 20 Mbps, 168-bit 3DES IPsec VPN throughput: 17 Mbps, 128-bit AES IPsec VPN throughput: 30 Mbps). One possibility is a MTU issue on the PIX outside interface. Have you try reducing that? It just might work.