The noob questions ...

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I would like to know if doing extra partitions for each wows would help reduce the loading times and lag ?

No. Extra partitions are still using the same physical disks, so essentially you're sharing the same read/write heads across all partitions. That won't help performance at all, and may result is worse performance if those heads have to thrash back and forth across disk areas since data will be stored in different locations. There's a way to improve performance on one disk though.. keep reading..

Is it possible to create different wow's shortcut for each instance ? Some shortcuts to start wow in "cheap graphics" mode when I do multi-box and a shortcut to start wow in "high graphic" mode when I solo.

Yes and no. You need separate config folders. Basically, you need to have multiple WoW folders, but instead of each having a data folder, you need to sym-link all the data folders to the first WoW install folder. There's a section in the Wiki on how to do this. It's easily the best way to improve performance on one PC with one hard drive (or one array of drives).

The very noob questions : except the keyclone software, is there any other software I need to install to get pip ? like maximizer ? octopus ?

From what I understand, KeyClone has a maximizer, so you don't need any other software to multibox on one machine.