Storage
IOPS
Input/output operations per second: a measure of how many individual reads and writes a storage volume can handle, key for database performance.
IOPS counts the individual read and write operations a disk can complete per second. It's the metric that predicts how a database or busy application will feel, because those workloads generate a flood of small, random I/O rather than a few big sequential transfers. High throughput in megabytes per second and high IOPS are different things, and databases care about the latter.
Many providers cap IOPS by tier or charge to provision more, which is where surprise slowness comes from. A volume might have plenty of space but a low IOPS ceiling that throttles a database under load. NVMe-backed storage delivers dramatically higher IOPS than older SSD or HDD volumes, which is why it matters for the write-heavy case.
If your app feels slow and the CPU looks idle, storage IOPS is a prime suspect. It's an easy thing to overlook when comparing plans that all advertise the same disk size.