Access to 320 kbs MP3's on NAS slow

I am having a bit of a challenge getting iTunes to access certain music files quickly. I have a lot of 320 kbs mp3s that I ripped in Windows Media Player years ago primarily because the encoder sounded better to me and I prefered how it organized my albums into artist> album heirarchy and put a folder.jpg file in the album for album art. Most (but not all) of these files open very slowly. Between a 15-30 second delay which is not really usable.

Some of the files open quickly. I have other files that are between 128 and 320 kbs and they mostly open within a second. But the majority of my library is 320kbs mp3's.

Not sure what to do as I have about a 400 gig library and I can access them quickly in all sofware other than iTunes but I need to use itunes to work with my iphones and ipods efficiently.

Are there any suggestions for optimizing my itunes library? I manually manage my music and I am in the process of tagging and renaming files and embedding album art. I was thinking I should ultimately rebuild my library since there will be a lot of dead links. Any suggestions on how to approach this process and also how I may troubleshoot the culprit of the extreme slowdown? Anything will help.

I use a Infrant Readynas NV with pretty recent firmware but the NAS doesnt seem to be the issue because as I mention some files open quickly in itunes, other dont.