Be sure that all Oracle processes, both background processes and user processes, have the same process priority. When you install Oracle, all background processes are given the default priority for your operating system. You should not change the priorities of background processes. You should also be sure that all user processes have the default operating system priority.
Assigning different priorities to Oracle processes may exacerbate the effects of contention. Your operating system may not grant processing time to a low priority process if a high priority process also requests processing time. If a high priority process needs access to a memory resource held by a low priority process, the high priority process may wait indefinitely for the low priority process to obtain the CPU, process, and release the resource.