Constant DISK_FULL_REFRESH 

const DISK_FULL_REFRESH: Duration;

How often the disk list and each volume’s capacity/free space are re-read. Every tick only refreshes I/O counters (the read/write rates need a delta per tick). A full refresh re-lists every mounted volume and, on macOS, asks each for its “available for important usage” capacity, which goes through CacheDelete: with a few DMGs mounted that was thousands of system calls and log lines a minute, for numbers that move slowly.