Slow Disk I/O (Read/Write) Speed

Hi,

I have VMWare Fusion installed on Mac (Version: 10.10.2) with PCIe SSD storage. Virtual machine is running CentOS 6. When virtual machine is not being used, the disk I/O (read/write speed) is perfect, 1,500 MB/s. When the virtual machine is starting to be even slightly used, the disk I/O drops down to under 50 MB/s.

Here is disk I/O test when the virtual machine is being used:

root@server1 [/home]# dd if=/dev/zero of=test bs=64k count=16k conv=fdatasync
16384+0 records in
16384+0 records out
1073741824 bytes (1.1 GB) copied, 22.901 s, 46.9 MB/s
root@server1 [/home]

VMWare Tools have been installed, disk space is pre-allocated, Time Machine/FileVault/Firewall are all disabled.

Why does the disk I/O speed decrease so much when the virtual machine is even slightly used? The virtual machine is used as a web server for hosting sites with Apache and MySQL.

Thank you.