There is also a possible bottleneck with vmdk type. By default all VMDKs are created in "zeroedthick" mode. It means that all the space is preallocated, but not wiped, so on first access to each block on vmdk ESX wipes block first. As a result you see degraded performance for some time.

zeroedthick (default) .
Space required for the virtual disk is allocated during creation. Any data remaining on the physical device is not erased during creation, but will be zeroed out at a later time during virtual machine read and write operations.

eagerzeroedthick
Space required for the virtual disk is allocated at creation time. Unlike with the zeroedthick format, the data remaining on the physical device is zeroed out during creation. Disks in this format might take much longer to create than other types of disks.

thick
Space required for the virtual disk is allocated during creation. This type
of formatting doesn.t zero out any old data that might be present on this allocated space.

---
VMware vExpert '2009
http://blog.vadmin.ru