Hey ArnisR,
I finally found the problem. In the last ESXi 5.5 and the 6.0 release they have replaced the hpvsa driver (HP vSphere ISO !!). With these drivers, I have experienced major problems with the data stores and thus the B120i (RAID) controller of the HP MicroServer Gen8.
The problem is caused by the last (HP) ISO's (from march);
- VMware ESXi 5.5.0-Update2-2403361-HP-550.9.2.27 Mar2015.iso
- VMware ESXi 6.0.0-2494585-HP-600.9.1.39 Mar2015.iso
I have found out (because I had an older ISO), the ISO VMware ESXi 5.5.0-Update2-2068190-HP-5.77.3-Nov2014.iso works well! With this ISO, all performance problems disappeared 
.
The difference between the ISO's is the following. The older ISO contains the storage driver scsi-hpvsa 5.5.0-88OEM. ISO(s) of March come with newer storage driver scsi-hpvsa 5.5.0-92OEM or scsi hpvsa-5.5.0-90. These drivers are the cause of the problems in the B120i controller!
When installing ESXi 5.5 or 6 from the last ISOs, you must manually install the older storage driver (SCSI hpvsa-5.5.0-88). I've tested this with the vib file scsi HPSA-5.5.0-88OEM.550.0.0.1331820.x86_64.vib. I then adjusted the HP ISO and equipped with the older driver. Now I can install ESXi 5.5 or 6 from the (customized) HP ISO, which includes the older driver.
I hope this helps!