In RAID 10 arrays, disk performance is crucial, especially if they are high-speed 15K RPM SAS HDDs, as each disk in the array is part of a mirrored pair that also stripes data with another pair. When replacing a disk, it's essential that the new disk matches the specifications of the others, especially in terms of rotational speed (RPM). If the replaced disk is slower, it can significantly reduce the Input/Output operations per second (IOPS) of the entire array. This is because all disks need to work in tandem, and the slowest disk can become a bottleneck. Thus, checking the RPM of the newly replaced disk is a sensible first step to ensure it matches the performance of the other disks in the array.