I have the same machine as you, purchased it in July 2009. Photoshop is usable in less than ten seconds. When it's running from my (pending RMA) Intel SSD it loads pretty much instantly.
I wonder if it's fragmentation of your drive or perhaps the number of fonts you have installed that's causing such a long load?
I have 312 fonts installed, don't know if that's a moderate or heavy load but there you go. I've seen worse. As for fragmentation, well maybe but isn't that supposed to be a non-issue with Macs? Before I switched to Mac, I listened to all these Baghdad Bobs in the Mac community who reassuringly stated that Macs are soooo maintenance-free and that they have a Mac from the year 1785 that's exactly as snappy as it was on the day they unboxed it and they've never ever performed any kind of maintenance. When the machine was brand new I only had CS3, but I don't recall ever having seen Photoshop load in mere seconds. Faster than CS5, certainly, but not by much. How can I determine if the drive is fragmented?
I upgraded to SL from Leopard last fall when SL was released (September?). I've done some maintenance such as repairing disk permissions a few times, two disk repairs, and manually deleted residual garbage from the library folders (root + user) due to lack of proper uninstallers, cleaned out the Safari cache... Otherwise I haven't touched the installation or performed any of those "archive/reinstall" or whatever they call it.
I rebooted and timed a few things...
Time for SL to load, from boot volume selection screen to logon prompt:
75 sec
Additional time for SL to load entirely (=from logon prompt to the moment that the hard drive stops chattering after loading login items, dock, desktop icons etc):
86 sec
Time for Safari to load (From clicking dock icon until window has appeared, the hard drive has stopped chattering and Safari is done loading the home page, favicons etc. and is 100% responsive):
46 sec
Time for Numbers (iWork '08) to load:
34 sec
Time for Flash CS5 to load (from clicking dock icon to application window and Welcome Screen appearing):
66 sec
Doing the same stuff in Windows happens in roughly half the time or less (Excel loads in 5 seconds, PS CS5 in 29 seconds etc), except the additional system boot time that happens after the logon prompt because Windows is loading the SuperFetch cache.
Edit: OK, I downloaded a demo of iDefrag and had it examine the boot volume. It says...
Fragmented files: 5062
Total files: 837170
Fragmentation 0.2%/0.6%
Avg. Frags: 1.02
Doesn't look too bad... right?
Anyway, sorry about the OT, don't mean to turn this into a 'fix my computer please' thread, but I just had to say something about the claim that Photoshop would load in 4 effing seconds.