How to ensure equal distribution of server resources between sites?

Question

Virtualization

Good day everyone. I ask for help in finding the optimal solution for the following problem:
There is a server (E3-1230v5/32gb/2Tb. Installed: PHP/MySQL/Apache/Debian-8). It hosts a dozen sites, each site has its own MySQL database (from 3-4gb in size), but the same php code is used. Each site has 2 to 15 concurrent users.
Usually the load on the server is acceptable, and everything works quickly for everyone. But, about once every 3 days, very resource-intensive operations lasting 10-15 minutes are launched on one of the sites. Then the remaining copies of the sites simply fall down.
The question is how to ensure a guaranteed server resource for everyone? Albeit at the cost of reducing the overall performance and increasing the time of complex operations.
I myself thought about creating a dozen virtual servers, but it seems to me that keeping a dozen copies of the OS and scripts is somehow not very ... Perhaps there is a better solution?

Answer 1

Why don't Monsieurs like containers, LXC for example?

Answer 2

10 sites with 2-15 users at the same time?
E3-1230v5/32gb/2Tb?
Brakes?
Flog the programmer with rods.