Naturally Qlik uses RAM during reload, as much RAM as it needs to hold the data being reloaded.
Qlik also uses CPU when reloading, as much CPU as it can get access to and utilize in order to complete the reload in as short period of time as possible.
RAM is working storage, CPU is the working power, Qlik needs both.
I'm not sure how you best would limit the CPU load during reload in order to release more CPU for the end-users.
Usual procedure is to have two servers, one back-end doing the reloads and one front-end serving the end-users. That way the reloads are separated from the end-users.
You can limit the CPU usage of the Engine by using throttling but that would affect the users also.
Throttling is to allow other processes than Qlik on the server that would otherwise be starved when Qlik uses all CPU.