Is there a WHM plugin mode or something like that which would limit the amount of resources a user on a dedicated server can use (CPU and RAM) or ideally something which gives certain users priority access to the resources needed?
Because my problem is that some websites sometimes use all the CPU available (through MySQL) and then the other sites are very slow...
The closest thing I could think of would be CloudLinux. It allows you to limit how much CPU/memory a site may use before it gets capped, however it does not limit MySQL usage, pretty much just PHP. You could limit every site on the server to a specific amount of open process/CPU besides the one you want to have more resources.