Apple - Why is download speed not increased using an SSD?

Probably the bottleneck is not your disk but your network.

A SATA2 hdd disk usually supports write speeds of up to 250/300MB/s. It is unlikely that you can reach those download speeds.

Upgrading your HD will not help with download speed. A better internet connection will help you have more download speed.

Your internet speed is a lot slower than the hard drive speed, so upgrading to a faster hard drive won't significantly increase download speed. It is your internet connection that's limiting your download speeds, not your harddrive.