The throughput of PIX 515E will depend on factors like the traffic that is flowing through it and if there is additional hardware cards installed or not. As a summary
Cleartext throughput: 188 Mbps
Concurrent connections: 130,000
168-bit 3DES IPsec VPN throughput: Up to 140 Mbps with VAC+ or 63 Mbps with VAC
128-bit AES IPsec VPN throughput: Up to 135 Mbps with VAC+
256-bit AES IPsec VPN throughput: Up to 140 Mbps with VAC+
Simultaneous VPN tunnels: 2000
Following link may help you
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_data_sheet09186a0080091b15.html