Windows 10: Slow RAID 0 volume (6 HDDs)

Having some speed issues on a HDD volume.
The volume in question is comprised of six 4TB WD red NAS (CMR) drives. Build specs below.

When I created the volume in the bios it was quite fast somewhere around 800-900mbs (read/write) benchmarked via CrystalDiskMark. I worked with it for awhile never noticed a speed decrease, filled it to %65 and noticed a bottle neck when processing RAW image sequences and RAW video. The volume was reading at around 200mb/s, writing around 500mb/s. After a little research I decided to back up all my data twice and delete some of my less important files, but before I did I thought to get a screen shot of my slow speeds. The reasonable speeds returned!, but my issues were far from over...







I have analyze/optimize set at weekly, and noticed that the speed increase came after an auto defrag. Great! I thought. Now that all my data is backed up twice. May as well clear some files off as I have around 8TB more data coming up by the end of the year. So I took the volume down to 32%, got the "files too large to recycle" dialogue box "delete anyway?" I selected yes, and also emptied the recycle bin. Did a speed test and my read/write speeds were cut in half! Removing data made it slower. I decided to try a manual analyze/optimize, restart. Same results. Shut down, unplugged the power, started up, same results. For most applications these speeds are fine, but they should be way faster and I need them to be. I work with 4k-8k raw video and raw image sequences (time-lapse). Lots of folders with thousands of 20-50mb RAW files.

Is there another way to empty the recycle bin, optimize, or write zeros to the empty space on the volume?
Thanks.





CPU: Intel CoreTM i7-7800X X-Series Processor, 3.5GHz w/ 8.25MB Cache
MB: Asus PRIME X299-A w/ DDR4-2666, 7.1 Audio, Dual M.2, Gigabit
LAN, 3-Way CrossFire / SLI
RAM: G.SKILL Ripjaws V Series 32GB DDR4 3200MHz Dual Channel (2x 16GB)
GPU: GeForce RTX2070 Gaming OC 8G
Power: EVGA SuperNOVA 850 G3 Modular Power Supply, 850W
Case: Fractal Design Define R6 Blackout Tempered Glass Edition E-ATX
Cooling: Noctua NH-D15 CPU Cooler

O.S is on: 2 X 250gb SSD Samsung 860 EVO
Media: 6 X 2TB HDD (7200 rpm) Hitachi (server grade).
Cashe: 1 X M.2 nVme500gb

:)