Qlikview is an in-memory product, this is how we're able to manipulate and display data quickly for users, it's all stored in memory/RAM. When Qlikview Server starts, it will allocate RAM according to the low working set limit set in the QMC. If you're running the default working set limits of 70/90, that means 70% of the 512 RAM in your server will be allocated to QVS, or about 358GB. Even though users aren't actively on the QMC and accessing QVWs, memory will still continue to be allocated to QVS.
Please take a look at the following article/video which explains Qlikview memory usage:
Srinivas, as far as memory consumption goes, the QVS process will consume memory resources up to the Low Working Set Limit specified in the QMC, and it will not release those resources until the memory usage goes above the Low Limit, this is how the working set limits work. Here is another article with a PDF attached that is similar to the video post in the article Josh gave you, but the PDF will likely go into more detail for you.
As far as CPU usage, I would confirm what process is actually consuming CPU cycles via Windows Task Manager, if it is the QVS process, and there are no users with open apps on the server, you can check this via QMC\Status\QVS Statistics area, I am not sure what could be going on, would have to see logs etc. You also did not mention what version you are running either, the most current supported releases are the following, if you are not on one of these, I would suggest updating to at least the one that matches your major release...
Shout back if you have further questions, but hopefully this will help you understand things better.
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