The issue is most likely related to caching. ESXi does not do any caching itself for data-integrity reasons and fully relies on the storage controller. The speed you see is not unusual for a storage controller which operates in write-trough mode, i.e. all writes have to be confirmed by the disk. If a controller operates in write-back mode (e.g. with battery or flash backed write cache) you may see an up to ~10-15 times better disk performance.
ndré