Hi Hanif,
let me tell you: you are not alone. Im also not happy with KM, specially not with the performance of the KM explorer und also not with Flex UI framework. I dont think pointing to 3rd party tools, (thats what people from Walldorf will probably tell you, if you talk to them about) cant be the final solution. A software company of the dimesions like SAP AG should be eligible to deliver own products with acceptable quality. KM is actually under "slight improvement", I guess they will let it die, as far I know there is even no product manager for KM actually.
But back on topic: from my experience, if you import large amount of KM data you have to ensure 2 things before you do it:
1. you have to ensure the tablespaces of underlying DB are big enough
2. you need double space on the files system for importing your stuff: since KMC are archives, they need to be extracted before import. This happens if you trigger the import routine. While importing EP will keep KMC archives as well as extracted stuff
regards