You haven't mentioned whether your RAID card has a cache or not. ESXi doesn't provide any software level caching like the Windows OS does, and performance on ESXi is generally accepted as being garbage on the cheaper cards which don't provide this facillity in hardware. It's certainly the most common cause of "my storage is slow" type complaints here.
It would also appear you have hard disks of the slower variety. Not being able to afford SSD is one thing - but using 15K drives is a huge benefit and at 1TB I don't expect your disks are of this style. Finally, you state that every VM is configured with 4 vCPU. With a reasonable number of virtual machines, this would lead to a high level of contention and odds are you could increase performance by lowering vCPU count, rather than just declaring every VM needs this much "for performance".
The only people RAID0 helps is people doing nothing but reading the results of benchmark software. There is no excuse to use it.