Trying to use SSD, it will improve the download speed obviously. 
Verification and write speed, after downloading, your computer need to verify the blocks individually. SSD greatly improves the speed. Download speed is dependent on your internet speed, not on hardware. Network speed is rarely the bottleneck unless you just have the bad luck of continually connecting to a really slow peer. Downloading 20 GB @ 5Mbps is only ~8 hours.
Downloading and verification aren't independent steps. To avoid being spoofed by other nodes your client will download a block, verify it, and then request the next block. The blockchain is downloaded, verified, and indexed one block at a time.