Hey nthorst,
I have the same problem I think...
I upgraded my ESXI 5.5.0 to ESXI 6.0 today and my top speed is 20 MB/s sometimes lower... Got RAID 10 with 4 hard disks...
How can I downgrade the storage driver?
Can you help me maybe?
Thank you very much!
Edit:
Found my old installation ISO: VMware-ESXi-5.5.0-1331820-HP-5.71.3-Sep2013.iso
This one worked fine. But I dont know how to install only that driver package 
Edit 2:
Installed now "scsi-hpvsa-5.5.0-88OEM.550.0.0.1331820.x86_64.vib" from here.
ESXI 6.0 running better now! Speed collapsing to 120 MB/s (I think it should be faster , maybe some of you guys have any idea) but it runs smooth again.