If you want to artificially constrain memory in order to test performance in low memory systems then this is a different use case entirely and I'd be surprised if there is not software available to do this for you.

Although you should bear in mind that computers with less memory probably have less everything else (including slower memory busses) so your best bet is to actually find a slower computer and test with that.

Most OSes will let you prioritize other processes over others (e.g Linux has "nice"), whether this is just for CPU preemption priority or whether it does other resources as well I'm not sure.

However I think most swapping systems will try and keep the most frequently used pages in RAM , so if one process has preemption priority over another this problem would solve itself as it's memory area would just get accessed more often.

Again this is kind of finger in the air stuff so anyone more familiar with OS design should feel free to correct me.