Apparently the update of Windows 10 Version 1809 has changed a device policy on Pegasus resulting in write performance issue.  The solution is below.

In Device Manager > Disk Drives > Promise Pegasus SCSI Disk Device > Properties > Policies > "Removal Policy" should be changed from "Quick Removal" to "Better Performance", and the "Enable Write Caching on the Device" should be selected.

After then, Pegasus write speed should be back to normal.