Someone like hic would be best placed to answer about the core usage.
You may find that IO is relatively low on the QVD load as the file is so well compressed it doesn't take much disk space (and therefore IO requirement). The work is around indexing and associating in RAM once the file is loaded.
This is my view on this topic.
Reading a stream of qvd data is not a cpu intensive operation. It is in fact single threaded, unless the script performs data transformations.
For QlikView a reload will only read one file/stream at the time per qv.exe or qvb.exe (Publisher reload engine).
Many parallel instances you will put a higher load on the disk.
The "core jumping" is, to my knowledge, under the operating systems control.