I don't know solution of problem, but I would check things step by step and try to localize the problem
1. Update BIOS, firmwares, ILO etc. Reset them all. Remove power completely from server for ~10 s (I know it is sounds weird, but I had problems after updates with booting/ILO on Microserver. Complete power-off resolved that.)
2. Install ESXi from scratch with latest HP image on internal SD card or USB. Without any disk attached. Check the booting time.
3. Attach one disk, check cables, make new RAID0 array from scratch on the whole disk. If problem appears, check with different manufacturer disk.
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