There is no correspondence between your operating - system’s reports on CPU usage and Adaptive Server’s internal “CPU busy” information. It is normal for an Adaptive Server to exhibit very high CPU usage while performing an I/O-bound task.
A multithreaded database engine is not allowed to block on I/O. While the asynchronous disk I/O is being performed, Adaptive Server services other user tasks that are waiting to be processed. If there are no tasks to perform, it enters a busy-wait loop, waiting for completion of the asynchronous disk I/O. This low-priority busy-wait loop can result in very high CPU usage, but because of its low priority, it is harmless.