PCIe or SATA attached SSD random access latency is around 0.2ms. Typical RAM latency is 100ns, maybe 200 in a NUMA cross-node access. That's a 3 orders of magnitude difference. Add another order of magnitude for network-attached SSD.

Bandwidth wise, a single DDR3 channel has around 10 GB/s (and a typical server has 4 to 8 of them). A single half duplex 10GE link (the most you can provision and effectively use on EC2) is 500 MB/s. So, generally 1 to 2 orders of magnitude.