I think your problem is a combination of things.
1. The wireless network may not be as fast as advertised.
2. iPhoto and iTunes operate in completely different ways
When you access iTunes to play a track, it streams the file to wherever and starts to play as soon as the first part of the file arrives on the remote machine.
When looking at the iTunes library or playlist, you are only looking at text. A tiny amount of data is required to show the contents of the library.
iPhoto, on the other hand, cannot do anything until ALL the file has arrived on the remote machine. These files are usually larger than the music files.
iPhoto, certainly with a large collection, takes some time to open, even when the library is stored on the internal HD.
You could try attaching the MBP directly to the NAS to see if that makes any improvement. If it does, then you know that the wireless is not that fast.
On another note, how are you using Time Machine on that drive without the drive being formatted at Mac OS X Extended(Journaled)?
I recently bought an external NAS drive so that I could free up my MBP from the increasing amount of photos and music. I chose a NAS drive so that I could access my music and photos wirelessly from my PS3 and also my MBP. My Iomega NAS can also accommodate Time machine backups, so my MBP is also backed up.
My iTunes library works fine across the wireless network. I can access it via my MBP and PS3, happy days!
The problem I have is with my iPhoto library. Its painfully slow, unusably slow when I try to access photos from MBP or PS3.
I have unfortunately resorted to not using iPhoto at all and simply manually storing my photos in 'event' labeled folders in the Photos share point on the NAS. This solution works fine but I'd like to be using iPhoto.
Is the problem because the iPhoto library is on a external drive? Or because its on a wireless, external drive? Or because its not a Mac formatted drive? Or all of the above?
If there's a way to get iPhoto to work on this NAS then some suggestions as to how it could be done would be very much appreciated.