What makes apps open quickly?

If I had to guess, it would be, in this order:

1. Amount of RAM
2. Hard Drive speed
3. Processor speed

 

It's a combination of everything, and small tweaks of each will yield different speeds.

 

It depens on the situation. If you just booted and haven't used the app the most important part will be the hard-drive. Because App's are stored on your hard-drive. If you used the app just recently and haven't rebooted or used other heavy app's it will still be in your ram. And therefore open quickly. When you have more ram the app will stay longer in your ram. Simply because OSX doesn't need to remove it for the benefit of other programs.

Processor speed is negligible when you talk about recent processors.

 

With your typical user out there with 4 gigs, it's rare RAM will make much difference unless they're doing RAM intensive stuff, such as working with VMs, encoding video, Photoshop, etc. Processor will matter some. By and large it's the data storage medium.

Go look at videos/benches displaying the difference the better SSDs make in this dept. It's not so much about data transfer rates but low latency.

 

With the speed of todays machines the sole factor is nearly always the memory subsystem. The speed of the harddrive, mostly, and the amount of physical ram available to load the application onto.