I had this problem too, cities were bad and shatrath during the day was an absolute nightmare.

However (!) there is a way to really help this if you meet the following criteria.
1) Running Windows Vista
2) Your problem stems from the fact that you have 5 seperate copies of the Warcraft directory

What you need to do is link all the Data and Cache directories in your clone Wow folders to the main one.

ex:
c:\Program Files (x86)\wow2\Data -> c:\Program Files (x86)\wow1\Data
c:\Program Files (x86)\wow2\Cache -> c:\Program Files (x86)\wow1\Cache

You do this with the 'mklink' tool.

1) Remove the Data and Cache directories in each clone wow folder (or rename them)
2) Open up the Command Prompt
3) Change directory to the clone wow
> cd c:\Program Files (x86)\wow2
4) Make the link
> mklink /D Data c:\Program Files (x86)\wow1\Data
> mklink /D Cache c:\Program Files (x86)\wow1\Cache
5) Repeat for each other clone directory

Doing this reduced my lag in shatrath from horrible to zero.

edit: The main reason I did this was when I got my epic flyers anytime I would fly over _anything_ I had a high chance of losing one, it doesn't happen anymore

:!: Concerns:
I haven't upgraded WoW yet with a patch, and because the files are split like this there may be issues. This is why I haven't deleted the data directories yet, so I can restore them if I have to.
Can some explain that, having problems following it.
I get 'The syntax of the comand is incorrect' error at step 4
Thanks