See: How to install applications to a separate hdd?.
Even though it is possible, Windows works fundamentally different than Linux. It is a good idea to learn the *NIX directory structure and work within it. If you are not familiar with Unix/Linux/Ubuntu, I would recommend that you leave things where they are.
Instead, make room for your operating systems and most frequently used software on your fastest drive. As you pointed out, it's better for software (including your OS) to be located on a fast drive.
Most linux software takes up a very small amount of space. Notable exceptions would be video games, but the platforms for very large games (ex. Steam) allow you to choose the destination for software without negatively affecting the UNIX directory structure.
The primary technical reason why your software runs better on solid state storage compared to a mechanical drive is because software routinely needs simultaneous access to lots of different files which are often stored in many different sectors located in different parts of the drive. A mechanical disk has to physically seek to access sectors whereas a solid state drive does not have this limitation. This makes the "random read speed" insanely faster on SSDs than mechanical hard drives.
Meanwhile, something like a several GB video file is more likely to be stored sequentially on the drive. SSDs are still typically faster at reading these files, but a mechanical disk is not necessarily slowed down by having to seek. You also typically load documents, pictures, movies, etc, "on demand" one at a time, rather than having to load a bunch of them all at the same time. You can always keep these files on any mounted file system that Ubuntu can read and write, so they will not need to take up space on your root file system if you do not want them there.
