I've deassembled nexuiz (NOT reverse engeneered) and I've seen that ir's made out of .ase , .tga and .cfg (configuration files) they were all (apart from nexuiz.exe and few .dll) packed as .zip and had the file extension changed into .pk3 so they were smaller and the program don't get confused. But doesn't packing slow down the game?

You deassembled Nexuiz? hehe, its a well known fact the pk3 is 'just' a renamed zipfile, games do that for a very long time now, it makes patching quite easy and also requires to only have one or a few files open at the same time, and decompressing stuff and loading BEFORE the real level starts is not a big deal.. that would have to be done also if it were not compressed.. that not a slowdown, rather a speedup and the compression allows to read small data and uncompress it on the fly..

thx

And btw to understand the code, you don't need to reverse engineer it, because you have the source code