I was wondering if I move my Unity3d project from HDD to SDD, how it will improve performance? There are two possible cases. First, if you already have your system (with Unity3d editor) installed on SSD (I hope you do) and a second case is when you have only HDD (I hope you will buy SSD asap). To figure that out I made some simple tests

I took a first computer with OS and programs installed on SSD and run all tests on a project which was located on HDD, then I moved the project to the SSD drive and did the same tests again.

SSD computer: Core i5-4590, 16Gb RAM, SSD KINGSTON HyperX 3K 120Gb, HDD Seagate Barracuda 7200.14 2TB

A second computer has only HDD drive, but faster CPU. I run tests only once.

HDD computer: Core i7-4770, 16Gb RAM, HDD Western Digital Blue 1TB

Here are results:

Now more about the project. At Novility, we are creating next-gen interactive training system (Novility HELP, coming soon at HITEC). Our project is almost 7 Gb big, so all tests were made with this pretty big project.

As you can see, there is not much difference if your system is installed on SSD, an overall performance of the whole system already improved. But if you migrate your system from HDD to SSD it will give you ~1,8x speedup in your Unity3d development. Besides all other performance improvements (faster program and system loading, UI response time, etc).

It is worth to have SSD as a main drive in your system, and for a very large files you can always buy cheap and spacious HDD.