Yes. When you do a lot of work quickly, it affects your performance on other jobs! Mload will run as fast as it can. If it is run at the same priority as other jobs, the resource will be shared between these other jobs and you may notice some degredation - but it will not block them out completely because there is still some I/O wait time and resource sharing. Use Priority Scheduler to control it; give your production users some priority over loading and testers.