I'm wondering if below stats are normal:
all my drives are WD green 3TB. i don't have cache drive setup. i'm interested in knowing if adding a cache drive will improve this stats significantly or not.
The "cache" drive improves "write" speeds to the array. It slows down read speeds (a tiny-tiny-bit)
Write speeds are improved only because the process of writing to the protected array is delayed to a later time. You files are NOT protected until copied to the protected array/
Read speeds are slowed because the cache drive must FIRST be scanned for any given file before scanning the protected data disks in the array. That takes a tiny bit of time.
To answer your question, the stats on the physical drives when reading them will not change at all. They have nothing to do with actual read and write speeds to the array. Most disks will not be able to sustain 100MB/s read speeds.