Particularly the description of Inactive memory.
This information is in RAM but it is not actively being used, it was recently used.
For example, if you've been using Mail and then quit it, the RAM that Mail was using is marked as Inactive memory. Inactive memory is available for use by another application, just like Free memory. However, if you open Mail before its Inactive memory is used by a different application, Mail will open quicker because its Inactive memory is converted to Active memory, instead of loading it from the slower drive.
I agree: unless your purge cache memory using the "purge" command in Terminal, or a third-party conveniently named Purge, whatever memory a recently closed app has used is still preempted by it, and remains available. The same goes in Windows, when DLLs remain in active memory throughout a whole session, and sometimes clog the system to a halt. Memory management in OS X is much better, though...
The only way not to experiment these slowdowns is to put your Mac to sleep instead of shutting it down. You can decide to hibernate it, too.
Thanks X423424X and FrenchToast, that sheds some light on why it's so much quicker to launch after the first time, but now for the inevitable follow-up question.
I'm now wondering why my apps take so long to launch the first time? What I mean is that while this has been the case for quite some time, it wasn't always the case, so I'm wondering what would have caused this?
Maybe my behaviour has changed - perhaps I used to always put the MacBook to sleep rather than shut it down.
Thanks again for your help!
To understand the why of this relative slowliness, you must understand that no application is really self-contained, or standalone, whatever the OS. All applications rely on so-called native code to function. Basically, some libraries (graphics, mainly, but not only) that are already installed on your machine, as part of the system, or installed by some third-party software.
On a cold boot, all this needed code has to be launched and installed in memory (physical and/or virtual) before the application core can be launched too. In your case, maybe some preference files are corrupted somewhat, or permissions need repair. You can try the following experiment: create a new standard account, give it admin rights, log in this new account and check how long it takes for your usual applications to launch. If they're quicker, then there's a problem with your main account; if it takes them the same amount of time to launch than in your other account, then you might consider adding some RAM on your machine, or free some space on your hard drive.
Or both. Preferably both...
I think before doing any expirementation boot from another drive or your installer dvd and run Disk Utility to repair/verify your boot drive (not repair permissions).
Also toss in a smc reset for luck!