Volume Latency is how long it takes for a storage device to complete a requested action. The action is either to read or write some data.
This subject has a great deal of complexity and varies widely from one server or VMS to another. So I will attempt to be short and concise and cover the majority of cases in very broad statements.
A properly functioning and healthy LIVE VIDEO storage device should complete an action in under 200ms. If it takes longer than 200ms you are likely having buffer overflow issues and losing video data before it can be written to disk.
The C: volume can take much longer but should in most cases have a respones time under 500ms. In some edge cases like copying large amounts of very small files it will take longer.