File Copy Performance Issues

Hi,

I have a HP Micro Server with a HP Raid card and 3 disks in a Raid 5 configuration.

ESXi is booting from USB.

I have create a Windows 2008R2 Server with a VMXNET3 Nic, create a folder and shared this on the 1GB network.

I have done a single 8GB file copy to the guest access a 1GB network - it did the first 5GB at 55MB/Second and then dropped to 22MB/Sec -

i then tried another large ISO file and it copied at 15MB/Second

Anyone know why this happens? or is there anything i can do to fix this issue..?

Thanks
Sam