Deleting files and slow speeds

So I have a wd ex4 with 4x3tb WD Red drives in RAID 5 and there is only about 8tb of usable storage but there is about 300gb left of space and it started slowing down, so i deleted a bunch of content about 1tb worth of stuff a few days ago but there only is still about 300gb free. How do i reclaim my storage space? How do i speed my system back up to normal?

I believe it’s just like windows in which if you have the recycle bin active you need to log in to the EX4.  Then go to settings and scroll down and clear the recycle bin. 

Yes you need to purge the recycle bin on occasion.