The SSP is where the CPU complex of the ASA 5585-X resides. The various ones you mentioned proviode 4, 8 16 and 24 cores respectively. See figure 2 here:
http://www.cisco.com/c/en/us/products/collateral/security/asa-5585-x-adaptive-security-appliance/whitepaper_C11-731802.html
They provide higher throughput as you move up the line. This is because the ASA software is multicore-aware (SMP in the image name stands for Symmetric MultiProcessor) and divides the workload across the cores.