A little question about transfer speeds

I have this lovely little Unraid server 4x18 TB (HDD) on the Array and 2x2 TB (SSD) in a pool

If I transfer lets say 30GB from various places to the various drives (2.5Gbit Ethernet) then I get different speeds:
Desktop -> Array (HDD) starts at 283MB/s and drops after a little time to 125MB/s
Desktop -> Pool (SSD) 300MB/S all the way (even with a 100GB transfer)
Pool -> Array 200MB/s constantly all the way

My question is then... WHY do the Desktop -> Array slow down MORE after it transfered some, but I don't see the transfer slow down when I go from Pool to Array. The Network clearly handles the speed to the Pool, so why not on the Array? It makes no sense to me... So any enlightenment?