It may not be writing the disk that is a bottleneck, but rather the “on demand” calculation of values to write.
Is the system sluggish for other tasks? Do you have space on your disk? If there is stuff in the Recycle bin, emptying the Recycle bin might help.
It might be useful to state the value of “Memory” in the “About SNAP” panel.
If you have AV software see if there is a a tool to display status of current scans and check the logs for anything related to your files. I’ve had problems in the past with AV Software failing while scanning large remote sensing data files, giving up, and restarting the scan over and over.
You can poke around in Task Manager to see if anything looks odd: CPU very busy or idle, memory usage, high page faults, etc.
Has the AV software been busy (in Processes tab, select both CPU and CPU Time for viewing)?
Event Viewer may give some hints about the underlying problem.